(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.

plural of active couple

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) A transitional structure, in an energetically excited state, that forms between the reactants of a chemical reaction and breaks down to form the products

(organic chemistry) Any of a wide range of techniques, often using catalysts, in which a carbon to hydrogen covalent bond of an organic compound is made to cleave in order to replace the hydrogen with a functional group.

Partially active

Alternative spelling of pseudoactive

(biology) Movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.