What is an expression in a given language that cannot be understood from the literal meaning of the words?

An idiom is a phrase or expression that conveys a meaning not deducible from the literal definitions of the individual words within it. For instance, expressions like "kick the bucket," which means to die, do not make sense when interpreted word-for-word. The meaning of an idiom often stems from cultural or contextual usage, making it difficult for someone unfamiliar with the language or culture to grasp its significance just by looking at the words alone.

Other terms like proverb, colloquialism, and metaphor have different roles in language. A proverb is a short, well-known saying that expresses a general truth or piece of advice, often deriving meaning from shared wisdom. A colloquialism is an informal expression used in everyday conversation, which might be regionally specific but does not necessarily obscure its meaning. A metaphor involves a comparison between two unrelated things to highlight a similarity, but unlike idioms, the metaphorical meaning can often be inferred from the context. Thus, idioms uniquely fit the description of expressions whose meaning cannot be understood through a direct interpretation of their component words.