过去时 is a grammatical term for the past tense — not the same as the time word 过去 (guòqù, 'the past') or the verb complement 过去 (guòqu, 'go over'). It's used in discussing verb tenses in grammar study, especially when learning Western languages that have explicit past-tense inflection (English, French, etc.). In Chinese, verbal aspect (了, 过, 在, 着) replaces tense, so 过去时 is primarily a metalanguage term.