A grammatical tense which expresses the past as an action which was still going on at the point in time described, say someone "was ...ing" something, as opposed to the already accomplished counterpart past perfect tense (where the action took place even further in the past)
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.