(mathematics) A branch of functional analysis, being the study of linear operators on function spaces.

(figuratively) To remain vigilant at all times, especially by remaining partially awake or sleeping only lightly so as to avoid being caught off guard.

(journalism) An article in which the author expresses their personal opinion on a given subject.

plural of operative word

Synonym of angle harp

plural of open harp

(military, psychiatry) The acute psychological trauma experienced in combat environments which can lead to combat fatigue.

The difference of electrical potential between two terminals of a device when disconnected from any circuit. The open-circuit voltage of a cell is its EMF (electromotive force).

(uncountable) A genre of opera with light, comedic themes (less risqué than those of the operetta) which was popular in 19th century Britain; especially the works of Gilbert and Sullivan.

A major category of Chinese opera, originating in southern China's Cantonese culture, and involving music, singing, martial arts, acrobatics, and acting.