mouth harp vs harmonica

mouth harp and harmonica both are nouns.

Word	Noun	Adjective	Verb	Adverb
mouth harp	Yes	No	No	No
harmonica	Yes	No	No	No

As nouns, mouth harp and harmonica are synonyms defined as:

mouth harp and harmonica: a small rectangular free-reed instrument having a row of free reeds set back in air holes and played by blowing into the desired hole

Other synonyms of mouth harp include harp, mouth organ.

mouth harp (noun)	harmonica (noun)
a small rectangular free-reed instrument having a row of free reeds set back in air holes and played by blowing into the desired hole	a small rectangular free-reed instrument having a row of free reeds set back in air holes and played by blowing into the desired hole