‘唇齿相依’ literally means ‘lips and teeth depend on each other’—a vivid idiom drawn from anatomy: lips protect teeth, while teeth support lip shape and function. Though both are separate parts, neither can thrive without the other. This image powerfully conveys deep, inseparable mutual dependence between two entities.

The phrase is used metaphorically to describe relationships where two parties rely on one another for survival, success, or stability—such as neighboring countries, business partners, or interlinked ecosystems. It carries a formal, literary tone and often appears in political discourse, economic analysis, or ecological writing. Unlike casual synonyms like ‘互相需要’, 唇齿相依 implies structural, almost biological necessity—not just preference or convenience.