Is the phrase "move over" an official English idiom? And if so, is it only in American English?

Is the phrase "move over" an official English idiom known worldwide? I would like to know:

1. Is it an official English idiom (not slang or colloquial)?
2. Is it known outside of the US (e.g. in the UK, Australia, India)?
3. Does it have any multiple meanings? Does it have any ambiguousness?
4. How likely it is for a non-native English speaker to know what this idiom means?
5. What is its official meaning? (is it "make way" or "move aside")

Can it mean "move toward"?

1. What is this "Official" of which you speak?
2. Yes. It's a perfectly common phrase in the UK, with variants such as "Budge over" or "Shift over" having the same meaning. I can't speak for other countries, but I have no reason to suspect that it's limited to the US and the UK.
3. Not really, no.
4. As per 2, I'd expect most native English speakers to recognise it, though I could be wrong.
5. Again, what do you mean by "official"? The English language doesn't have a governing body in the way that the Académie française tries to be for French. Dictionaries follow usage rather than defining it. I understand "Move over" as meaning "Move aside," such as to make space on a sofa or give the speaker access to a computer keyboard.

"Move over" on its own doesn't imply motion towards anything, just (short) motion away. You could say "Move over to [somewhere]," but that's a different construction.

Move over (or move aside) is a phrasal verb that means "adjust one's position to make room for someone else." It is used in both British English, and American English.

People who don't speak English as first language should not have any problems to understand the phrase, if they understand what move means. In some other languages it is not necessary to add over to move to suggest to make room for another person.

Move over is not slang phrase.

In American English, move over could also have two additional meanings.

• to stop doing something in order to let someone else do it
• to start to do something in a different way

She moved over to let me pass.
They seem to expect older musicians to move over so that the younger ones can get a chance.
Most companies moved over to direct payments years ago.

I wouldn't say move over is actually slang or colloquial. But it's normally used as a curt injunction to someone to get out of your way (or line of sight), which in effect makes it somewhat rude, and "slangy" by association.

Step aside, please [sir] is preferrable if you want to be firm without giving offense. Certainly that's what I'd expect from, say, a policeman speaking to a member of the public in relevant circumstances.