a person or thing that indicates.

a pointing or directing device, as a pointer on the dial of an instrument to show pressure, temperature, speed, volume, or the like.

an instrument that indicates the condition of a machine or the like.

an instrument for measuring and recording variations of pressure in the cylinder of an engine.

Chemistry .

a substance, as litmus, that indicates the presence or concentration of a certain constituent.

a substance often used in a titration to indicate the point at which the reaction is complete.

Ecology . a plant, animal, or species that indicates, by its presence in a given area, the existence of certain environmental conditions.

noun

barometer , beacon , clue , dial , gauge , guide , hint , index , mark , meter , omen , pointer , signal , symbol , warning , badge , evidence , indication , manifestation , note , signification , stamp , symptom , token , witness , (as in a hotel) annunciator , bellwether , forerunner , sign