What do you call a phrase that has a meaning that is not deducible from the individual words?

An idiom is a phrase where the collective meaning is not directly derived from the individual words that make it up. For example, the expression "kick the bucket" does not literally relate to kicking a bucket, yet it is widely understood to mean someone has died. This characteristic of idioms makes them unique, as they often reflect cultural meanings and nuances that cannot be interpreted through the literal definition of the words involved.

In contrast, a metaphor is a figure of speech that makes an implicit comparison between two unlike things without using "like" or "as," while a simile does use those terms for comparison. Hyperbole is an exaggerated statement not meant to be taken literally, used for emphasis or effect. Each of these other terms pertains to ways of expressing meaning or imagery, but none encapsulates the particular quality of a phrase's meaning being unique from its constituent words, which defines an idiom.