A moment is a particular point in time. Some moments you remember forever: like the moment you earned your driver's license or the moment you learned that you passed Organic Chemistry.
A moment is a short, but unspecified, amount of time. At Thanksgiving, you are supposed to take a moment to be grateful for friends and family. Or maybe you have been asked to observe a "moment of silence" for a national tragedy. Moment can also mean "importance." If you say something is "of little moment," then you are saying it doesn't matter.
Primary Meanings of moment
1. 
n 
an indefinitely short time

2. 
n 
a turning force produced by an object acting at a distance (or a measure of that force)

3. 
n 
the nth moment of a distribution is the expected value of the nth power of the deviations from a fixed value

Full Definitions of moment
1
n an indefinitely short time
“wait just a
moment”
 Types:

New York minute, blink of an eye, flash, heartbeat, instant, jiffy, split second, trice, twinkling, wink
a very short time (as the time it takes the eye to blink or the heart to beat)
 Type of:

time
an indefinite period (usually marked by specific attributes or activities)
n a particular point in time
“the
moment he arrived the party began”
 Types:
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climax, culmination
the decisive moment in a novel or play

eleventh hour, last minute
the latest possible moment

moment of truth
a crucial moment on which much depends

moment of truth
the moment in a bullfight when the matador kills the bull

pinpoint
a very brief moment

time
a suitable moment

psychological moment
the most appropriate time for achieving a desired effect

high time
the latest possible moment

occasion
the time of a particular event
 Type of:

point, point in time
an instant of time
n at this time
“she is studying at the
moment”
 Synonyms:
 here and now, present moment
n having important effects or influence
“virtue is of more
moment than security”
 Synonyms:
 consequence, import
 Antonyms:

inconsequence
having no important effects or influence
 Types:

matter
(used with negation) having consequence

hell to pay
dire consequences
 Type of:

significance
the quality of being significant
2
n a turning force produced by an object acting at a distance (or a measure of that force)
 Types:
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moment of inertia
the tendency of a body to resist angular acceleration

moment of a couple
given two equal and opposite forces, the product of the force and the distance between them

dipole moment
the moment of a dipole

electric dipole moment
the dipole moment in an electric dipole

magnetic dipole moment
(physics) a current loop gives rise to a magnetic field characteristic of a magnetic dipole
 Type of:

force
(physics) the influence that produces a change in a physical quantity
3
n the nth moment of a distribution is the expected value of the nth power of the deviations from a fixed value
 Types:
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second moment
the expected value of the square of the deviations of a random variable from the point of origin

variance
the second moment around the mean; the expected value of the square of the deviations of a random variable from its mean value

standard deviation
the square root of the variance

covariance
(statistics) the mean value of the product of the deviations of two variates from their respective means
 Type of:

statistic
a datum that can be represented numerically