An idiom is defined as:

An idiom is indeed a common expression that doesn't make sense if taken literally. This definition captures the essence of idiomatic phrases, which often convey meanings that are culturally understood rather than directly inferred from the individual words. For instance, when someone says "it's raining cats and dogs," the literal interpretation fails to reflect the actual meaning, which is that it is raining heavily. Understanding idioms is an important aspect of language as they often provide insight into cultural nuances and can add richness and color to communication. The other options do not accurately define idioms; they either describe different linguistic concepts or suggest a level of formality not inherent to idiomatic expressions.