(law) An activity that is so dangerous that any person engaging in it will be held strictly liable for harm caused by engaging in the activity, no matter what precautions were taken to prevent harm; common examples include using explosives, radiation, and poisonous chemicals, and handling wild animals.

(grammar, uncountable) the form in which the subject of a verb carries out some action / (grammar, countable) a form in a particular language used to express the active voice

(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.

(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 activation analysis

The increase in rate of a chemical reaction caused by the presence of a catalyst. / An amount of catalyst, expressed as the increase of the rate of its reaction.

(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.

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