indicator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that indicates, especially.
• noun A pointer or an index.
• noun An instrument used to monitor the operation or condition of an engine, furnace, electrical network, reservoir, or other physical system; a meter or gauge.
• noun The needle, dial, or other registering device on such an instrument.
• noun Chemistry 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 two or more substances by means of a characteristic change, especially in color.
• noun Ecology An indicator species.
• noun Any of various statistical values that together provide an indication of the condition or direction of the economy.

from The Century Dictionary.

• noun In railroad signaling, a device for informing the leverman in a signal-cabin that a train is about to start from the station and indicating which track it will take; in its broadest sense, any appliance for displaying, in the signal-cabin, the condition of a track or of all the tracks in a yard, the position of the signals, semaphores, switches, and signal-lamps, the trains at rest, or moving, or about to enter or leave any block, etc. The indicator may be a number on a drop-plate, a disk or banneret, or a miniature signal-arm, and it may give information by its appearance or disappearance or by its position. It may also give a signal by means of a bell. An indicator may be operated from a distant station or cabin by a push-button, or it may be automatic, or it may be controlled by a train through a track-circuit.
• noun In mining, an appearance of the surface of the ground which shows the presence of a mineral underneath.
• noun Naval: An apparatus used in conjunction with a transmitter, operated by mechanical or electrical means for signaling orders from a central position to the various places on a war-ship at which the orders are to be executed. The transmitter is manipulated by the operator in the central station or in the conning-tower to show any desired order, and the same order is shown on one or more indicators connected to the transmitter by wires or shafting. A range-indicator shows the range of the object at which the guns are to be fired; a battle-order indicator shows various orders such as ‘commence firing,’ ‘load with shell,’ etc. An apparatus to show at a convenient Point the position of any mechanism: as, a revolution-indicator to show the direction of revolution of the main engines; a rudder -indicator, to show the position of the helm or rudder; a turret-indicator, to show the position of the turret guns with reference to the fore-and-aft line of the ship; etc.
• noun One who indicates or points out; that which points out, directs, or reports, as a grade-post on a railroad, the pointer on a steam-gage, etc.
• noun Specifically— A steam (cylinder) pressure-gage. It is an apparatus for recording the variations of pressure or vacuum in the cylinder of a steam-engine. The accompanying cut represents a type of the indicator. The pipe with the stop-cock is screwed to the cylinder so that when the cock is opened the pressure of the steam within may enter the cylinder above, press the piston upward against the action of a spring constructed to give a definite resistance in pounds per square inch, and cause the lever-arm to rise and mark on the hollow cylinder at the left a vertical trace, the altitude of which measures the pressure. A card or a sheet of paper may be fitted to this cylinder, and the trace be made on the paper. The hollow cylinder is free to revolve, if drawn by the loose cord hanging from it. To operate the indicator, the cord is connected with some moving part of the engine so that a single stroke of the piston causes the cylinder and the card to revolve once (the return being secured by a spring) as the pencil makes one mark. Since the pencil-mark is timed to one stroke of the engine, the resulting curved line on the card gives a graphic report of the pressure or vacuum of the steam during one complete stroke. Such graphic curves are called indicator-diagrams, the marked card being called an indicator-card. See indicator-diagram, under diagram
• noun The dial and pointer of a signal-telegraph used on private lines, where rapidity of delivery of the messages is not important. It consists of a dial having the letters of the alphabet printed upon it, and a pointer that traverses the circle, pausing before the letters of the word transmitted, thus spelling out the message. See telegraph.
• noun In a microscope, an arrangement for marking the position of a particular object in the field of view. Quekett's indicator was a steel finger connected with the eyepiece.
• noun In mining, an arrangement by means of which the position of the cage in the shaft is known to the man in charge of the winding-engine.
• noun In the theory of numbers, the exponent of that power of any number less than and prime to any modulus, which power is the least power of the same number congruous to unity.
• noun In anatomy, the extensor indicis, a muscle which extends the forefiuger, as in the act of pointing. It arises from the back of the ulna, and is inserted into the index-finger, which can thus be straightened independently of the other fingers.
• noun In ornithology: A honey-guide; a species of the genus Indicator or family Indicatoridæ.
• noun [capitalized] The typical and leading genus of Indicatoridæ, established by Vieillot in 1816. I. major and I. minor are examples. See Indicatoridæ.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, shows or points out.
• noun (Mach.) A pressure gauge; a water gauge, as for a steam boiler; an apparatus or instrument for showing the working of a machine or moving part.
• noun (Steam Engine) An instrument which draws a diagram showing the varying pressure in the cylinder of an engine or pump at every point of the stroke. It consists of a small cylinder communicating with the engine cylinder and fitted with a piston which the varying pressure drives upward more or less against the resistance of a spring. A lever imparts motion to a pencil which traces the diagram on a card wrapped around a vertical drum which is turned back and forth by a string connected with the piston rod of the engine. See Indicator card (below).
• noun A telltale connected with a hoisting machine, to show, at the surface, the position of the cage in the shaft of a mine, etc.
• noun (Mech.) The part of an instrument by which an effect is indicated, as an index or pointer.
• noun (Zoöl.) Any bird of the genus Indicator and allied genera. See Honey guide, under Honey.
• noun (Chem.) That which indicates the condition of acidity, alkalinity, or the deficiency, excess, or sufficiency of a standard reagent, by causing an appearance, disappearance, or change of color, as in titration or volumetric analysis.
• noun the figure drawn by an engine indicator, by means of which the working of the engine can be investigated and its power calculated. The Illustration shows one form of indicator card, from a steam engine, together with scales by which the pressure of the steam above or below that of the atmosphere, corresponding to any position of the engine piston in its stroke, can be measured. Called also indicator diagram.
• noun a telegraph in which the signals are the deflections of a magnetic needle, as in the trans-Atlantic system.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A pointer or index that indicates something.
• noun A meter or gauge.
• noun The needle or dial on such a meter.
• noun chemistry Any of many substances, such as litmus, used to indicate the concentration of a substance, or the degree of a reaction.
• noun ecology A plant or animal whose presence is indicative of some specific environment.
• noun economics A measure, such as unemployment rate, which can be used to predict economic trends.
• noun UK A trafficator.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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
• 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
• noun a device for showing the operating condition of some system
• noun a signal for attracting attention