earphone vs earpiece

earphone and earpiece both are nouns.

Word	Noun	Adjective	Verb	Adverb
earphone	Yes	No	No	No
earpiece	Yes	No	No	No

As nouns, earphone and earpiece are synonyms defined as:

earphone and earpiece: electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear

Other synonyms of earphone include headphone, phone.

earphone (noun)	earpiece (noun)
electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear	electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear