An idiom is a phrase which through common usage has come to have a metaphorical meaning that may not be clear form the literal meanings of the constituent words (so being 'over the moon' has nothing to do with location. altitude, or, indeed, the moon).
Idioms may potentially be a barrier to understanding for second language learners as the meaning of an idiom can often not be deduced by translation as an idiom is often specific to a particular language.
Read about communicating science through idioms
