‘底线’ (dǐxiàn) literally means 'bottom line' — combining 底 (dǐ), meaning 'bottom' or 'base', and 线 (xiàn), meaning 'line'. Figuratively, it refers to the minimum acceptable limit — a boundary beyond which one will not go, whether in behavior, ethics, negotiation, or personal standards. It often conveys firmness, principle, or non-negotiable terms.

This word is widely used in everyday conversation, business, politics, and interpersonal relationships. For example, someone might say their 'moral bottom line' when refusing to lie or cheat, or a company may state its 'price bottom line' in negotiations. Unlike abstract concepts like 'principle' (原则), 底线 emphasizes a concrete, often unstated but understood threshold — crossing it triggers consequences or ends cooperation. It carries a sense of seriousness and finality, making it common in both formal and informal contexts where boundaries matter.