An idiom is a sequence of words which have a different meaning in group from the one they have separately.  For example, when you say "something smells fishy" you don't really mean that there is a horrible smell, but that there is something in a situation that seems to be strange.

There is a big difference between a collocation and an idiom. A collocation is the way words combine in a language to produce natural speech and writing. For example when you say "pay attention", it could be "give attention, or put attention" but it is not, it is pay attention because it is the natural … Read more