I'm Feeling Under the Weather

Omair Imat says, when do we use the phrase under the weather when you're feeling sick? He's also says, when does an expression become an idiom? I don't even know what the official difference is. If we looked at the dictionary, I kind of think of them as basically the same thing. Maybe there's some technical difference between an expression, a saying and an idiom and slang,. but they're all basically the same things.