A grammatical form (often a verb form) that refers to an event, transaction, occurrence, or object that happened (or had happened), or existed, at some time before now (the applicable reference time).
A grammatical form (often a verb form) that is used in a conditional expression to refer to an event, transaction, occurrence, or object that is hypothetical and often counterfactual.
A feature of the verb which denotes viewing the event the verb describes as a completed whole, rather than from within the event as it unfolds. For example, "she sat down" as opposed to "she was sitting down". Since the focus is on the completion of what is expressed by the verb, this aspect is generally associated with the past and future tenses. This term is often used interchangeably with aorist aspect. This is not to be confused with the perfect tense.