(chemistry) An acid-base indicator.

(aviation) An instrument that displays an aircraft's current airspeed.

(chemistry) any substance used to classify another, often by changing colour

(aviation, UK, military) An instrument that displays attitude of an aircraft in pitch and roll.

The imperfect tense in the indicative mood

(grammar) The present tense in the indicative mood.

plural of ordinal indicator

A sign adjacent to a numeral denoting that it is an ordinal number, rather than a cardinal number.

plural of pH indicator

A float used by anglers to tell when a fish has taken the bait