Is there an idiom, word or phrase that means one or the other? By that, I mean, you can only choose between one or the other. You have two choices, not three or more. Just one or the other. I think neither-or is used, but I am wondering if there are more expression, word, etc.
Either/or means one or the other.
Neither/nor means not one and not the other.
The most common adage for "you can only choose one" is probably:
You can't have your cake and eat it too.
(Note that "have" here means possess, whereas usually "to have cake" means to eat cake. You can't eat your cake and still have it. You must choose one.)
Another common but less charming adage says:
You can't have it both ways.