The beginning or duration or completion or repetition of the action of a verb WordNet 3.0 & Open English WordNet

In linguistics, aspect is a grammatical category that expresses how an action, event, or state, as denoted by a verb, extends over time. Wikipedia

Grammatical category denoted by a verb Wikidata

A grammatical quality of a verb which determines the relationship of the speaker to the temporal flow of the event the verb describes; whether the speaker views the event from outside as a whole, or from within as it is unfolding. OmegaWiki

A grammatical quality of a verb which determines the relationship of the speaker to the internal temporal flow of the event which the verb describes, or whether the speaker views the event from outside as a whole, or from within as it is unfolding. Wiktionary

Quality of a verb. Wiktionary (translation)

The aspect of a verb phrase or other predicate – its temporal structure or relationship to the time of use – marked through any of various grammatical elements, as distinguished from lexical aspect. Wiktionary