It is my understanding that harp is a slang term for Mouth Harp or French Harp, both of which ( and several others ) refer to a "Harmonica". The term "Harmonica" actually refers to a free reed instrument of which their are multiple kinds, not just the "harmonica" type we think of. The number of "holes" or "reeds per hole" doesn't seem to play any part of it. However, since we all grow up with different colloquial phrase, I'm sure some of us have heard a harp has only one reed per hole or a harmonica have more than one reed per hole or variation there of.
