An idiom is an expression whose meaning is different than the literal meaning of the words. Idioms give flavor, and personality to your writing.

WHO SAID IT BETTER

That test was easy.

that test was a piece of cake.

It's raining really hard.

it's raining cats and dogs.

Idioms become part of a dialect, or the way locals express themselves. If you tell someone in California that you are going to hang ten, they know you are going surfing. If someone from England tells you that your outfit is brilliant, they don’t mean that your shirt is smart.