(chemistry, of a crystal or compound) Exhibiting optical activity.

capable of lowering the surface tension of a liquid

The specific compound in a natural or man-made mixture that is responsible for the mixture's activity.

(physics, meteorology) The boundary between two different air masses, or a portion thereof, which produces appreciable cloudiness and precipitation and is usually accompanied by significant shifts in wind direction.

(volcanology, geology) A volcano that has erupted in the last 10,000 years, although not necessarily venting lava or gases now.

In set dances with more than one couple dancing, the couple doing the more complicated movement during any given portion of the dance. The role of active couple may pass from one couple to another. In many quadrilles, certain figures will have two active couples at any one time; in long set dances, quite often every other couple or every third couple is the active couple.

plural of active couple

(military) A state in which person in the military performs military duties on a full-time basis, as opposed to reserve, where one only occasionally performs military duties.

The process of attending carefully to what a speaker is saying, involving such techniques as accurately paraphrasing the speaker's remarks.

The real component of the power of an alternating current circuit; the product of the voltage and that part of the current that is in phase with the voltage