'曾经' (céngjīng) is an adverb meaning 'once' or 'formerly', indicating that an action or state occurred at some indefinite time in the past but no longer does. Though composed of two characters — 曾 (céng, 'already' or 'previously') and 经 (jīng, 'to experience' or 'to go through') — together they form a fixed compound expressing completed past experience, not current reality. It emphasizes that something was true or happened before, often implying contrast with the present.

This word frequently appears in narrative, reflective, or comparative contexts — for example, when recalling childhood habits, past jobs, or former relationships. Unlike time-specific markers like '昨天' (yesterday) or '去年' (last year), '曾经' points to an unspecified moment or period in the past and carries a subtle nuance of distance or change. It typically precedes verbs or adjectives and cannot stand alone as a noun or subject.