Which term describes an expression whose meanings cannot be inferred from its individual words?

The chosen term is correct because an idiom is a figure of speech where the collective meaning cannot be deduced from the meanings of the individual words that compose it. For instance, the phrase "kick the bucket" does not mean to literally kick a bucket; instead, it figuratively means to die. Understanding idioms requires familiarity with the expression in its entirety, as the meanings are often cultural or contextual rather than logical.

In contrast, a literal expression conveys a straightforward meaning directly derived from its components. A metaphor compares two unlike things without using "like" or "as," often revealing deeper meanings but still not fitting the criteria of an expression whose meaning is not inferred from its words. A phrase is simply a small group of words that work together but doesn’t necessarily demonstrate the unique, non-literal characteristic of an idiom.