mouth organ vs harmonica

mouth organ and harmonica both are nouns.

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

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

mouth organ 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 organ include harp, mouth harp.

mouth organ (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