If you're familiar with the verb to "indicate," then indicator should be no problem. An indicator points to, suggests, or shows something. The arrow on a street sign is an indicator telling you which way to go.

"It's not the heat, it's the humidity!" In other words, temperature is only one indicator of how it feels outside. Air-pressure, humidity, wind-chill, and cloud cover are other important indicators of weather conditions. You might hear a lot from newscasters about "economic indicators." These are figures, such as the rate of unemployment or inflation, that indicate where we're headed economically. Good or bad, we need reliable indicators if we're going to recover economically. Not all the indicators say the same thing: some say we're going to be OK sooner than others.

Definitions of indicator
  1. noun
    a device for showing the operating condition of some system
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    an indicator that announces which electrical circuit has been active (as on a telephone switchboard)
    cursor, pointer
    (computer science) indicator consisting of a movable spot of light (an icon) on a visual display; moving it allows the user to point to commands or screen positions
    the circular graduated indicator on various measuring instruments
    fuel gauge, fuel indicator
    an indicator of the amount of fuel remaining in a vehicle
    indicator provided by the stationary arm whose shadow indicates the time on the sundial
    level, spirit level
    indicator that establishes the horizontal when a bubble is centered in a tube of liquid
    indicator lamp, pilot lamp, pilot light
    indicator consisting of a light to indicate whether power is on or a motor is in operation
    an indicator as on a dial
    an indicator having a graduated sequence of marks
    test paper
    paper impregnated with an indicator for use in chemical tests
    indicator consisting of a line at the highwater or low-water limits of the tides
    a rotating pointer on the face of a timepiece
    electronic stylus, light pen
    (computer science) a pointer that when pointed at a computer display senses whether or not the spot is illuminated
    mason's level
    a level longer than a carpenter's level
    a slender pointer for indicating the reading on the scale of a measuring instrument
    vernier, vernier scale
    a small movable scale that slides along a main scale; the small scale is calibrated to indicate fractional divisions of the main scale
    type of:
    an instrumentality invented for a particular purpose
  2. noun
    (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction
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    absorption indicator
    an indicator used in reactions that involve precipitation
    acid-base indicator
    an indicator that changes color on going from acidic to basic solutions
    Molisch reaction, Molisch test, Molisch's test, alpha-naphthol test
    biochemical indicator of the presence of carbohydrates in a solution; if carbohydrates are present a violet ring is formed by reaction with alpha-naphthol in the presence of sulfuric acid
    oxidation-reduction indicator
    an indicator that shows a reversible color change between oxidized and reduced forms
    a laxative used in many preparations under various trade names; also used as an acid-base indicator in titrations involving weak acids and strong bases because it is brilliant red at high alkalinity and colorless below pH 8
    congo red
    a red-brown azo dye especially as a chemical pH indicator (congo red is red in basic and blue in acidic solutions)
    methyl orange
    an azo dye used as an acid-base indicator; used for titrations involving weak bases
    litmus, litmus test
    a coloring material (obtained from lichens) that turns red in acid solutions and blue in alkaline solutions; used as a very rough acid-base indicator
    bromophenol blue, bromphenol blue, tetrabromo-phenolsulfonephthalein
    a dye used as an acid-base indicator
    bromothymol blue, bromthymol blue
    a dye used as an acid-base indicator
    fluorescein, fluoresceine, fluorescent dye, resorcinolphthalein
    a yellow dye that is visible even when highly diluted; used as an absorption indicator when silver nitrate solution is added to sodium chloride in order to precipitate silver chloride (turns pink when no chloride ions are left in solution and negative fluorescein ions are then absorbed)
    type of:
    color, coloring material, colour, colouring material
    any material used for its color
  3. noun
    a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time
    synonyms: index, index number, indicant
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    Dow-Jones Industrial Average
    an indicator of stock market prices; based on the share values of 30 blue-chip stocks listed on the New York Stock Exchange
    Standard and Poor's Index
    a broadly based stock market index
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    BMI, body mass index
    a measure of someone's weight in relation to height; to calculate one's BMI, multiply one's weight in pounds and divide that by the square of one's height in inches; overweight is a BMI greater than 25; obese is a BMI greater than 30
    business index
    a statistical compilation that provides a context for economic or financial conditions
    leading indicator
    one of 11 indicators for different sections of the economy; used by the Department of Commerce to predict economic trends in the near future
    price index, price level
    an index that traces the relative changes in the price of an individual good (or a market basket of goods) over time
    short account
    the aggregate of short sales on an open market
    stock index, stock market index
    index based on a statistical compilation of the share prices of a number of representative stocks
    retail price index
    an index of changes in retail prices
    producer price index, wholesale price index
    an index of changes in wholesale prices
    CPI, consumer price index, cost-of-living index
    an index of the cost of all goods and services to a typical consumer
    type of:
    a statement or assertion of verified information about something that is the case or has happened
  4. noun
    a signal for attracting attention
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    point of reference, reference, reference point
    an indicator that orients you generally
    bench mark, benchmark
    a surveyor's mark on a permanent object of predetermined position and elevation used as a reference point
    a mark showing the boundary of a piece of land
    lubber line, lubber's line, lubber's mark, lubber's point
    a fixed line on a ship's compass indicating its heading
    mark, target
    a reference point to shoot at
    white line
    a white stripe in the middle of a road to mark traffic lanes
    bright-colored; a float attached by rope to the seabed to mark channels in a harbor or underwater hazards
    type of:
    sign, signal, signaling
    any nonverbal action or gesture that encodes a message
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