We would say "The monetary unit in American accounting is the USD." Since "units" are countable, we put a "the" in front of "unit". Since "accounting" is uncountable, we don't put a "the" in front of it. Here, we are choosing between several options: it is the USD, not the RMB, NTD, or VND. Any time you are choosing between multiple choices that actually exist in the world, use "is". My favorite animal *is* the dog. Use "mean" to explain what a person intends to say when they say a sentence,. You can also use it when you are defining a term. When she said "we should just be friends", she meant "I don't love you anymore". The word "spectacular" means "great", not "good". The term "monetary unit" means the kind of money that a country uses.

You could use either 'is' or 'means' This sense of 'mean' (to have value or importance of) is rather informal: OK for everyday speech, but if this is for an exam question or essay use 'is'.