In Catholicism, a cardinal is a highranking bishop. In math, you use cardinal numbers to count. A cardinal rule is one that is central and should not be broken.
Okay, that's a lot of definitions. How exactly are they related? In all cases, cardinal means central or essential. It's a cardinal principle that you use it to describe words of behavior like rule or sin. In the Church, cardinals form the central governing body, and in math the cardinal numbers (one, two, three) are the numbers you learn and use first.
Primary Meanings of cardinal
1. 
n 
a variable color averaging a vivid red

2. 
adj 
serving as an essential component

3. 
adjn 
being or denoting a numerical quantity but not order
the number of elements in a mathematical set; denotes a quantity but not the order

4. 
n 
(Roman Catholic Church) one of a group of more than 100 prominent bishops in the Sacred College who advise the Pope and elect new Popes

5. 
n 
crested thickbilled North American finch having bright red plumage in the male

Full Definitions of cardinal
1
2
3
adj being or denoting a numerical quantity but not order
“cardinal numbers”
 Synonyms:

0, zero
indicating the absence of any or all units under consideration

nonzero
not involving zero

1, ane, i, one
used of a single unit or thing; not two or more

2, ii, two
being one more than one

3, iii, three
being one more than two

4, four, iv
being one more than three

5, five, v
being one more than four

6, half dozen, halfdozen, six, vi
denoting a quantity consisting of six items or units

7, seven, vii
being one more than six

8, eight, viii
being one more than seven

9, ix, nine
denoting a quantity consisting of one more than eight and one less than ten

10, ten, x
being one more than nine

11, eleven, xi
being one more than ten

12, dozen, twelve, xii
denoting a quantity consisting of 12 items or units

13, thirteen, xiii
being one more than twelve

14, fourteen, xiv
being one more than thirteen

15, fifteen, xv
being one more than fourteen

16, sixteen, xvi
being one more than fifteen

17, seventeen, xvii
being one more than sixteen

18, eighteen, xviii
being one more than seventeen

19, nineteen, xix
being one more than eighteen

20, twenty, xx
denoting a quantity consisting of 20 items or units

21, twentyone, xxi
being one more than twenty

22, twentytwo, xxii
being two more than twenty

23, twentythree, xxiii
being three more than twenty

24, twentyfour, xxiv
being four more than twenty

25, twentyfive, xxv
being five more than twenty

26, twentysix, xxvi
being six more than twenty

27, twentyseven, xxvii
being seven more than twenty

28, twentyeight, xxviii
being eight more than twenty

29, twentynine, xxix
being nine more than twenty

30, thirty, xxx
being ten more than twenty

31, thirtyone, xxxi
being one more than thirty

32, thirtytwo, xxxii
being two more than thirty

33, thirtythree, xxxiii
being three more than thirty

34, thirtyfour, xxxiv
being four more than thirty

35, thirtyfive, xxxv
being five more than thirty

36, thirtysix, xxxvi
being six more than thirty

37, thirtyseven, xxxvii
being seven more than thirty

38, thirtyeight, xxxviii
being eight more than thirty

39, ixl, thirtynine
being nine more than thirty

40, forty, twoscore, xl
being ten more than thirty

41, fortyone, xli
being one more than forty

42, fortytwo, xlii
being two more than forty

43, fortythree, xliii
being three more than forty

44, fortyfour, xliv
being four more than forty

45, fortyfive, xlv
being five more than forty

46, fortysix, xlvi
being six more than forty

47, fortyseven, xlvii
being seven more than forty

48, fortyeight, xlviii
being eight more than forty

49, fortynine, il
being nine more than forty

50, fifty, l
being ten more than forty

51, fiftyone, li
being one more than fifty

52, fiftytwo, lii
being two more than fifty

53, fiftythree, liii
being three more than fifty

54, fiftyfour, liv
being four more than fifty

55, fiftyfive, lv
being five more than fifty

56, fiftysix, lvi
being six more than fifty

57, fiftyseven, lvii
being seven more than fifty

58, fiftyeight, lviii
being eight more than fifty

59, fiftynine, ilx
being nine more than fifty

60, lx, sixty, threescore
being ten more than fifty

61, lxi, sixtyone
being one more than sixty

62, lxii, sixtytwo
being two more than sixty

63, lxiii, sixtythree
being three more than sixty

64, lxiv, sixtyfour
being four more than sixty

65, lxv, sixtyfive
being five more than sixty

66, lxvi, sixtysix
being six more than sixty

67, lxvii, sixtyseven
being seven more than sixty

68, lxviii, sixtyeight
being eight more than sixty

69, ilxx, sixtynine
being nine more than sixty

70, lxx, seventy
being ten more than sixty

71, lxxi, seventyone
being one more than seventy

72, lxxii, seventytwo
being two more than seventy

73, lxxiii, seventythree
being three more than seventy

74, lxxiv, seventyfour
being four more than seventy

75, lxxv, seventyfive
being five more than seventy

76, lxxvi, seventysix
being six more than seventy

77, lxxvii, seventyseven
being seven more than seventy

78, lxxviii, seventyeight
being eight more than seventy

79, ilxxx, seventynine
being nine more than seventy

80, eighty, fourscore, lxxx
being ten more than seventy

81, eightyone, lxxxi
being one more than eighty

82, eightytwo, lxxxii
being two more than eighty

83, eightythree, lxxxiii
being three more than eighty

84, eightyfour, lxxxiv
being four more than eighty

85, eightyfive, lxxxv
being five more than eighty

86, eightysix, lxxxvi
being six more than eighty

87, eightyseven, lxxxvii
being seven more than eighty

88, eightyeight, lxxxviii
being eight more than eighty

89, eightynine, ixc
being nine more than eighty

90, ninety, xc
being ten more than eighty

91, ninetyone, xci
being one more than ninety

92, ninetytwo, xcii
being two more than ninety

93, ninetythree, xciii
being three more than ninety

94, ninetyfour, xciv
being four more than ninety

95, ninetyfive, xcv
being five more than ninety

96, ninetysix, xcvi
being six more than ninety

97, ninetyseven, xcvii
being seven more than ninety

98, ninetyeight, xcviii
being eight more than ninety

99, ic, ninetynine
being nine more than ninety

100, c, hundred, one hundred
being ten more than ninety

101, ci, hundred and one, one hundred one
being one more than one hundred

105, cv, one hundred five
being five more than one hundred

110, cx, one hundred ten
being ten more than one hundred

115, cxv, one hundred fifteen
being five more than one hundred ten

120, cxx, one hundred twenty
being ten more than one hundred ten

125, cxxv, one hundred twentyfive
being five more than one hundred twenty

130, cxxx, one hundred thirty
being ten more than one hundred twenty

135, cxxxv, one hundred thirtyfive
being five more than one hundred thirty

140, cxl, one hundred forty
being ten more than one hundred thirty

145, cxlv, one hundred fortyfive
being five more than one hundred forty

150, cl, one hundred fifty
being ten more than one hundred forty

155, clv, one hundred fiftyfive
being five more than one hundred fifty

160, clx, one hundred sixty
being ten more than one hundred fifty

165, clxv, one hundred sixtyfive
being five more than one hundred sixty

170, clxx, one hundred seventy
being ten more than one hundred sixty

175, clxxv, one hundred seventyfive
being five more than one hundred seventy

180, clxxx, one hundred eighty
being ten more than one hundred seventy

190, one hundred ninety, xcl
being ten more than one hundred eighty

200, cc, two hundred
being ten more than one hundred ninety

300, ccc, three hundred
being one hundred more than two hundred

400, cd, four hundred
being one hundred more than three hundred

500, d, five hundred
denoting a quantity consisting of 500 items or units

1000, k, m, one thousand, thousand
denoting a quantity consisting of 1,000 items or units

ten thousand
denoting a quantity consisting of 10,000 items or units

hundred thousand
(in Roman numerals, C written with a macron over it) denoting a quantity consisting of 100,000 items or units

million
(in Roman numerals, M written with a macron over it) denoting a quantity consisting of 1,000,000 items or units

billion
denoting a quantity consisting of one thousand million items or units in the United States

billion
denoting a quantity consisting of one million million items or units in Great Britain

trillion
one million million in the United States

trillion
one quintillion in Great Britain

zillion
very large indeterminate number
 Antonyms:

ordinal
being or denoting a numerical order in a series

zero
indicating an initial point or origin

zeroth
preceding even the first

1st, first
indicating the beginning unit in a series

2d, 2nd, second
coming next after the first in position in space or time or degree or magnitude

3rd, tertiary, third
coming next after the second and just before the fourth in position

4th, fourth, quaternary
coming next after the third and just before the fifth in position or time or degree or magnitude

5th, fifth
coming next after the fourth and just before the sixth in position

6th, sixth
coming next after the fifth and just before the seventh in position

7th, seventh
coming next after the sixth and just before the eighth in position

8th, eighth
coming next after the seventh and just before the ninth in position

9th, ninth
coming next after the eighth and just before the tenth in position

10th, tenth
coming next after the ninth and just before the eleventh in position

11th, eleventh
coming next after the tenth and just before the twelfth in position

12th, twelfth
coming next after the eleventh and just before the thirteenth in position

13th, thirteenth
coming next after the twelfth in position

14th, fourteenth
coming next after the thirteenth in position

15th, fifteenth
coming next after the fourteenth and just before the sixteenth in position

16th, sixteenth
coming next after the fifteenth in position

17th, seventeenth
coming next after the sixteenth in position

18th, eighteenth
coming next after the seventeenth in position

19th, nineteenth
coming next after the eighteenth in position

umpteenth, umptieth, umteenth
last in an indefinitely numerous series

20th, twentieth
coming next after the nineteenth in position

21st, twentyfirst
coming next after the twentieth in position

22nd, twentysecond
coming next after the twentyfirst in position

23rd, twentythird
coming next after the twentysecond in position

24th, twentyfourth
coming next after the twentythird in position

25th, twentyfifth
coming next after the twentyfourth in position

26th, twentysixth
coming next after the twentyfifth in position

27th, twentyseventh
coming next after the twentysixth in position

28th, twentyeighth
coming next after the twentyseventh in position

29th, twentyninth
coming next after the twentyeighth in position

30th, thirtieth
coming next after the twentyninth in position

31st, thirtyfirst
coming next after the thirtieth in position

32nd, thirtysecond
the ordinal number of thirtytwo in counting order

33rd, thirtythird
the ordinal number of thirtythree in counting order

34th, thirtyfourth
the ordinal number of thirtyfour in counting order

35th, thirtyfifth
the ordinal number of thirtyfive in counting order

36th, thirtysixth
the ordinal number of thirtysix in counting order

37th, thirtyseventh
the ordinal number of thirtyseven in counting order

38th, thirtyeighth
the ordinal number of thirtyeight in counting order

39th, thirtyninth
the ordinal number of thirtynine in counting order

40th, fortieth
the ordinal number of forty in counting order

41st, fortyfirst
the ordinal number of fortyone in counting order

42nd, fortysecond
the ordinal number of fortytwo in counting order

43rd, fortythird
the ordinal number of fortythree in counting order

44th, fortyfourth
the ordinal number of fortyfour in counting order

45th, fortyfifth
the ordinal number of fortyfive in counting order

46th, fortysixth
the ordinal number of fortysix in counting order

47th, fortyseventh
the ordinal number of fortyseven in counting order

48th, fortyeighth
the ordinal number of fortyeight in counting order

49th, fortyninth
the ordinal number of fortynine in counting order

50th, fiftieth
the ordinal number of fifty in counting order

55th, fiftyfifth
the ordinal number of fiftyfive in counting order

60th, sixtieth
the ordinal number of sixty in counting order

64th, sixtyfourth
the ordinal number of sixtyfour in counting order

65th, sixtyfifth
the ordinal number of sixtyfive in counting order

70th, seventieth
the ordinal number of seventy in counting order

75th, seventyfifth
the ordinal number of seventyfive in counting order

80th, eightieth
the ordinal number of eighty in counting order

85th, eightyfifth
the ordinal number of eightyfive in counting order

90th, ninetieth
the ordinal number of ninety in counting order

95th, ninetyfifth
the ordinal number of ninetyfive in counting order

100th, centesimal, hundredth
the ordinal number of one hundred in counting order

101st, hundredandfirst
the ordinal number of one hundred one in counting order

105th, hundredandfifth
the ordinal number of one hundred five in counting order

110th, hundredandtenth
the ordinal number of one hundred ten in counting order

115th, hundredandfifteenth
the ordinal number of one hundred fifteen in counting order

120th, hundredandtwentieth
the ordinal number of one hundred twenty in counting order

125th, hundredandtwentyfifth
the ordinal number of one hundred twentyfive in counting order

130th, hundredandthirtieth
the ordinal number of one hundred thirty in counting order

135th, hundredandthirtyfifth
the ordinal number of one hundred thirtyfive in counting order

140th, hundredandfortieth
the ordinal number of one hundred forty in counting order

145th, hundredandfortyfifth
the ordinal number of one hundred fortyfive in counting order

150th, hundredandfiftieth
the ordinal number of one hundred fifty in counting order

155th, hundredandfiftyfifth
the ordinal number of one hundred fiftyfive in counting order

160th, hundredandsixtieth
the ordinal number of one hundred sixty in counting order

165th, hundredandsixtyfifth
the ordinal number of one hundred sixtyfive in counting order

170th, hundredandseventieth
the ordinal number of one hundred seventy in counting order

175th, hundredandseventyfifth
the ordinal number of one hundred seventyfive in counting order

180th, hundredandeightieth
the ordinal number of one hundred eighty in counting order

190th, hundredandninetieth
the ordinal number of one hundred eightyfive in counting order

200th, twohundredth
the ordinal number of two hundred in counting order

300th, threehundredth
the ordinal number of three hundred in counting order

400th, fourhundredth
the ordinal number of four hundred in counting order

500th, fivehundredth
the ordinal number of five hundred in counting order

1000th, thousandth
the ordinal number of one thousand in counting order

millionth
the ordinal number of one million in counting order

billionth
the ordinal number of one billion in counting order

trillionth
the ordinal number of one trillion in counting order

quadrillionth
the ordinal number of one quadrillion in counting order

quintillionth
the ordinal number of one quintillion in counting order

nth, nth
last or greatest in an indefinitely large series
 show more antonyms...
n the number of elements in a mathematical set; denotes a quantity but not the order
 Synonyms:
 cardinal number
 Types:

absolute frequency, frequency
the number of observations in a given statistical category

googol
a cardinal number represented as 1 followed by 100 zeros (ten raised to the power of a hundred)

googolplex
a cardinal number represented as 1 followed by a googol of zeros (ten raised to the power of a googol)
 Type of:

number
a concept of quantity involving zero and units
4
n (Roman Catholic Church) one of a group of more than 100 prominent bishops in the Sacred College who advise the Pope and elect new Popes
 Examples:

Roberto Francesco Romolo Bellarmine
Italian cardinal and theologian (15421621)

Cesare Borgia
Italian cardinal and military leader; model for Machiavelli's prince (14751507)
 Types:

dean
(Roman Catholic Church) the head of the College of Cardinals
 Type of:

bishop
a senior member of the Christian clergy having spiritual and administrative authority; appointed in Christian churches to oversee priests or ministers; considered in some churches to be successors of the twelve Apostles of Christ