Indicator: a light named for its message, not its mechanism

To most English speakers, an indicator is a sign or measurement of something else: a stock market indicator, a health indicator, a speedometer needle that indicates engine revs. The word names the thing by what it reveals, not by what it is. A thermometer indicates temperature; a pressure gauge indicates pressure.

In automotive terminology, especially in British and Commonwealth English, an indicator is the turn signal light itself, the amber-colored lamp mounted on the front and rear quarter panels of a vehicle. When the driver activates the turn lever, the indicator flashes on and off at a rate of around 1.5 flashes per second. The term is metonymic: the light is named for the information it conveys (a turn or lane change) rather than for its construction (a bulb, lens, and housing). An indicator does not measure or display information passively; it actively broadcasts intention to other road users.

American English defaults to turn signal or directional signal, technical terms that name the device by its primary function. These names anchor themselves in what the apparatus does mechanically. The indicator, by contrast, floats free from mechanism and fastens onto meaning. This distinction parallels how British technical vocabulary often prioritizes the communicative or functional outcome, whereas American usage leans toward mechanical description.

The term emerged as automotive lighting became standardized in the mid-20th century. The flashing light was a new safety mechanism, and because its sole purpose was to indicate (warn, signal, broadcast) the driver's next move to surrounding traffic, the word indicator attached itself to the device and stuck, especially in markets where English retained older naming conventions. The device itself remained unchanged; the name simply privileged the semantic event over the electrical event.