大门 literally means 'big door' — 大 (dà) means 'big' or 'large', and 门 (mén) means 'door' or 'gate'. Together, they refer specifically to the main entrance of a building, compound, or institution — not just any door, but the principal, often prominent, access point. It’s commonly used for gates of schools, temples, government buildings, traditional courtyard homes (siheyuan), or even large residential compounds.

In everyday speech, 大门 carries a sense of formality, centrality, and sometimes symbolic importance — e.g., 'crossing the main gate' can imply entering a new phase (like starting school). Unlike 普通的门 (ordinary door) or 房门 (room door), 大门 is outward-facing and publicly visible. It’s rarely used for interior doors or small private entrances; those are typically called 门, 入口, or specific terms like 后门 (back door).