Is a idiom the same as a collection? I am guessing it is.

Also I was wondering about how valid idioms are in proper English. Most seem to be slang,  'A bird in the hand is worth two in the bush' or 'just what the doctor ordered' are many examples of idioms that would not be acceptable when writing English formally.

However, there are a few that are acceptable English 'abide by a decision' is one - taken from a Idioms dictionary - but is it really a idiom or just a sentence because I could write 'accept the decision' and the meaning would still be the same. I think?

Are idioms more important in American English then UK English?

Hope someone understands the questions and if they answer thanks for your reply!

Skittles

“Also I was wondering about how valid idioms are in proper English. Most seem to be slang,  'A bird in the hand is worth two in the bush' or 'just what the doctor ordered' are many examples of idioms that would not be acceptable when writing English formally. ”

I think of slang as the fare of the Urban Dictionary.  These expressions tend to come and go.  Much current street talk was unknown forty years ago, but most idioms date back many decades, if not hundreds of years.