indicator

[in-di-key-ter]
noun
1.
a person or thing that indicates.
2.
a pointing or directing device, as a pointer on the dial of an instrument to show pressure, temperature, speed, volume, or the like.
3.
an instrument that indicates the condition of a machine or the like.
4.
an instrument for measuring and recording variations of pressure in the cylinder of an engine.
5.
Chemistry.
a.
a substance, as litmus, that indicates the presence or concentration of a certain constituent.
b.
a substance often used in a titration to indicate the point at which the reaction is complete.
6.
Ecology. a plant, animal, or species that indicates, by its presence in a given area, the existence of certain environmental conditions.

Origin:
1660–70; < Medieval Latin indicātor, equivalent to Latin indicā(re) to indicate + -tor -tor

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
indicator (ˈɪndɪˌkeɪtə)
 
n
1.  See economic indicator something that provides an indication, esp of trends
2.  a device to attract attention, such as the pointer of a gauge or a warning lamp
3.  an instrument that displays certain operating conditions in a machine, such as a gauge showing temperature, speed, pressure, etc
4.  a.  a device that records or registers something, such as the movements of a lift, or that shows information, such as arrival and departure times of trains
 b.  (as modifier): indicator light
5.  Also called: blinker a device for indicating that a motor vehicle is about to turn left or right, esp two pairs of lights that flash when operated or a pair of trafficators
6.  Also called: dial gauge a delicate measuring instrument used to determine small differences in the height of mechanical components. It consists of a spring-loaded plunger that operates a pointer moving over a circular scale
7.  chem
 a.  a substance used in titrations to indicate the completion of a chemical reaction, usually by a change of colour
 b.  a substance, such as litmus, that indicates the presence of an acid or alkali
8.  ecology Also called: indicator species
 a.  a plant or animal species that thrives only under particular environmental conditions and therefore indicates these conditions where it is found
 b.  a species of plant or animal whose well-being confirms the well-being of other species in the area

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

indicator in·di·ca·tor (ĭn'dĭ-kā'tər)
n.

  1. One that indicates, especially a pointer or an index.

  2. An instrument used to monitor the operation or condition of an engine, an electrical network, or another physical system; a meter or gauge.

  3. The needle, dial, or other registering device on such an instrument.

  4. Any of various substances, such as litmus or phenolphthalein, that indicate the presence, absence, or concentration of another substance or the degree of reaction between substances by means of a characteristic change, especially in color.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
indicator   (ĭn'dĭ-kā'tər)  Pronunciation Key 
A chemical compound that changes color and structure when exposed to certain conditions and is therefore useful for chemical tests. Litmus, for example, is an indicator that becomes red in the presence of acids and blue in the presence of bases.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

indicator

any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. An example is the substance called methyl yellow, which imparts a yellow colour to an alkaline solution. If acid is slowly added, the solution remains yellow until all the alkali has been neutralized, whereupon the colour suddenly changes to red

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
That's because the mood in the market is often regarded as a contrarian
  indicator of future activity.
Rankings are essentially one-dimensional, since each indicator is considered
  independently.
Bullfrogs are amazing and they are an indicator of the health of their
  ecosystems.
Unemployment statistics have always been a trailing indicator of current
  economies.
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