迅速 (xùn sù) is an adjective meaning 'swift' or 'rapid', often describing speed in action, change, or response. It combines two characters: 迅 (xùn), meaning 'quick' or 'hasty', and 速 (sù), meaning 'speed' or 'velocity'. Together they form a compound that emphasizes both urgency and efficiency — more formal and literary than common alternatives like 快 (kuài). It’s frequently used in written Chinese, news reports, official documents, and formal speech.

The word typically modifies verbs (e.g., 迅速增长 — 'grow rapidly') or serves as a predicate (e.g., 反应很迅速 — 'the response is swift'). While it can occasionally function adverbially ('do something rapidly'), it’s not used before verbs without structural support (e.g., no *迅速跑; instead say 迅速地跑 or more naturally, 迅速行动). It conveys a sense of controlled, purposeful speed — not just quickness, but timely effectiveness.