headphone vs earpiece

headphone and earpiece both are nouns.

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

As nouns, headphone and earpiece are synonyms defined as:

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

Other synonyms of headphone include earphone, phone.

headphone (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