Yóulái is a noun meaning 'origin' or 'source'—it refers to how something came into being, where it began, or the historical or logical roots of a person, custom, place, or phenomenon. The character yóu (由) means 'by means of', 'from', or 'due to', while lái (来) means 'to come'. Together, they literally suggest 'how it came (to be)' or 'where it comes from'—a concise, elegant way to point to provenance or genesis.

This word is commonly used in formal or descriptive contexts: explaining the history of festivals, naming conventions, geographical features, or cultural practices. It often appears after a noun (e.g., 这个习俗的由来, 'the origin of this custom') and rarely stands alone. Unlike colloquial alternatives like 来源 (source), yóulái carries a subtle nuance of historical development or narrative background, making it frequent in textbooks, documentaries, and heritage descriptions.