Something done (usually as opposed to something said) WordNet 3.0 & Open English WordNet

In philosophy, an action is an event that an agent performs for a purpose, that is guided by the person's intention. Wikipedia

Something which is done by a person Wikipedia Disambiguation

Something an agent can do or perform Wikidata

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. Wiktionary