(music): The mechanism, that is the set of moving mechanical parts, of a keyboard instrument, like a piano, which transfers the motion of the key to the sound-making device.
(law) A charge or other process in a law court (also called lawsuit and actio).
(mathematics) A mapping from a pairing of mathematical objects to one of them, respecting their individual structures. The pairing is typically a Cartesian product or a tensor product. The object that is not part of the output is said to act on the other object. In any given context, action is used as an abbreviation for a more fully named notion, like group action or left group action.
The event or connected series of events, either real or imaginary, forming the subject of a play, poem, or other composition; the unfolding of the drama of events.
(art, painting and sculpture) The attitude or position of the several parts of the body as expressive of the sentiment or passion depicted.