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We have this device broken at work, for several days now. But I do not know how to refer to it.
By I have this feeling that they are not 100% interchangeable: sometimes, one is more suitable than the other.
The example tap water vs faucet water might not be the best, since "tap water" is a noun / noun phrase / idiom(atic) in itself.
It might be that one is UK, and the other US English? Or one is a part of the other (guess: a faucet is a part of a tap)?