Idiom – A phrase that means something different than the literal meaning of the words. (i.e. if you're feeling 'over the moon' you're not literally over the moon itself.)
The idiot told his mum (idiom) he would run three times around the block after she said he needed to go the extra mile to succeed in life. He hadn't realised her words weren't to be taken literally.
Examples of idiom in a sentence
The phrase 'flying high' to describe ones mood is an idiom.
The child found it hard to understand the idiom his mother used.
An idiom can cause confusion when directly translated into other languages.