内衣 (nèi yī) literally means 'inner clothing' — 内 (nèi) means 'inside' or 'inner', and 衣 (yī) means 'clothing' or 'garment'. Together, they refer to garments worn next to the skin, under outer clothing, for hygiene, support, comfort, or modesty. This term covers a broad category including bras, briefs, boxers, undershirts, and thermal layers.

In daily life, 内衣 is a neutral, standard term used in stores, advertisements, medical contexts, and casual conversation. It’s more formal and inclusive than colloquial terms like 小裤衩 (xiǎo kùchā, 'little underpants') or 胸罩 (xiōngzhào, 'bra'). Unlike English 'underwear', which can sound vague or humorous depending on context, 内衣 is consistently appropriate across registers — from shopping online to discussing laundry habits or health guidelines.