Expression is an expression whose meaning cannot be
guessed from the meanings of the words that make it up.
You cannot guess the
meaning of an idiomatic expression even if you know all the
words in an expression.
idiomatic expression: A can of worms
A can of worms has
nothing to do with a can or with worms. It means "TROUBLE"
This is another
famous idiomatic expression:
Have butterflies in the stomach
If you have
butterflies in your stomach, it means that you are worried or
you feel nervous about something you are going to do.
For more expressions
see the page: Idioms