Which word refers to a phrase established by usage as having a specific meaning?

The term that refers to a phrase established by usage as having a specific meaning is "idiom." Idioms are expressions that cannot be understood simply by looking at the individual words within them, as their meanings often differ from the literal interpretations of the words. For example, the phrase "kick the bucket" is commonly understood to mean "to die," but the literal meaning doesn't convey that idea at all. Idioms rely on cultural context and shared understanding, which is why they are so significant in language.

The other options have distinct meanings: "expression" is a broader term that encompasses a wide variety of sayings, phrases, and sentences, not limited to those with established figurative meanings. "Metaphor" refers specifically to a figure of speech that makes a comparison between two unlike things, which can create vivid images or help convey deeper meanings, but does not necessarily imply a fixed or established meaning in a phrase. "Colloquialism" pertains to informal words or phrases used in everyday conversation, which may vary by region but are not necessarily fixed phrases with a specific meaning like idioms. Understanding these nuances helps clarify why "idiom" is the correct choice in this context.