昨日 literally means 'the day before today' — 昨 (zuó) means 'previous' or 'last', and 日 (rì) means 'day'. Together, they form a formal or literary term for 'yesterday'. It is commonly used in written Chinese, news reports, historical narratives, official documents, and formal speech. While it carries the same core meaning as the more colloquial 昨天 (zuó tiān), 昨日 sounds more refined, solemn, or poetic, and appears less frequently in casual conversation.

This word functions syntactically as a time noun and typically appears at the beginning or end of a sentence to indicate when an action occurred. It can be modified by adverbs like 刚 (gāng, 'just') or 已经 (yǐjīng, 'already'), but unlike 昨天, it rarely takes measure words or classifiers. Its tone and register make it especially appropriate for recounting past events with gravity or elegance — for instance, in literature, memorial speeches, or journalistic headlines.