indicator

One that indicates, especially.

A pointer or an index.

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

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

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.

An indicator species.

Any of various statistical values that together provide an indication of the condition or direction of the economy.

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.

In mining, an appearance of the surface of the ground which shows the presence of a mineral underneath.