Sewage that has been aerated and agitated in order to promote the growth of beneficial microorganisms that break down organic matter.

(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.

(chemistry) activated carbon

(chemistry) A very absorbent form of finely powdered carbon; used in purifying gases, and as an antidote for many poisons.

(chemistry, physics) The energy required to initiate a reaction. For example, the flame from the fuse of a firecracker provides a small initial amount of energy, after which the explosive reaction proceeds by itself, releasing a considerably larger quantity of energy. A small push given to a stable but top-heavy object may cause it to fall over; the potential energy released during the fall was present in the system all along but could not be realized as long as the object was upright and balanced.

(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 activation analysis

(chemistry) Any of several similar radiochemical techniques of analysis in which a subatomic particle or gamma ray is captured by a nuclide to form a second stable nuclide which is then measured.

plural of active couple