Idiom

An idiom is a phrase or a sentence, which meaning is different than the individual words would imply, not a literal meaning.

Some examples: 

-"so long" does not mean that it is long, it means "good-bye";

- "to have butterflies in one's stomach" means "to be nervous";

- "to have a chip on one's shoulder" means "being upset over something that have happened in the past".