An idiomatic expression is a group of words
with a figurative meaning that is the same as their literal meaning
with a figurative meaning that is different from their literal meaning
that implies something negative about the subject
that implies something positive about the subject

An idiomatic expression is a group of words WITH A FIGURATIVE MEANING THAT IS DIFFERENT FROM THEIR LITERAL MEANING.

Idioms are used in everyday conversations in the English language. For example, hold your tongue. The figurative meaning is to stop talking, not the literal meaning of to hold your tongue :)