Definition -
(1) A method of classifying verbs by their relationship to time, e.g. whether or not the verb indicates an act that has an implied ending or not. For example, when you eat a grape, there’s an implied endpoint when the grape has been eaten; however, when you sit there’s no implied endpoint. (Source)

(2) The state of completion of a verb derived from its meaning, as opposed to this being derived from its morphology.

(3) The inherent aspect that a word has because of the way it is used in the lexicon. For example, the verb work has an inherently unbounded aspect, which means that the speaker must actively constrain it, e.g. I worked for an hour yesterday.

Etymology -
The term means "action type" in German.

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