A tense used in some languages (such as French and Italian) in narrative, and in spoken language primarily when reading narrative or in storytelling, for completed actions in the past or in speaking of the dead. It is semantically the same as the simple past in many languages, including English.
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)