Which of the following best describes an idiom?

An idiom is best described as a cultural expression that has a specific meaning which is often not deducible from the individual words in the phrase. For example, the expression "kick the bucket" refers to dying, and its meaning cannot be understood by simply interpreting the words literally. This cultural aspect is crucial, as idioms often reflect the traditions, values, or social norms of a particular group, making them unique to certain languages or regions. Other options do not capture the essence of an idiom: a phrase with a literal meaning would not fit the nature of an idiom, a new verb form does not relate to the concept of idioms, and a common grammatical rule refers to syntax rather than meanings derived from specific expressions. Thus, option B accurately encapsulates the unique characteristics of idioms.