earplug vs earpiece

earplug and earpiece both are nouns.

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

As nouns, earpiece is a hypernym of earplug; that is, earpiece is a word with a broader meaning than earplug:

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

Other hypernyms of earplug include earphone, headphone, phone.

earplug (noun)	earpiece (noun)
a plug of cotton, wax, or rubber that is fitted into the ear canal for protection against the entry of water or loud noise	electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear
an earphone that is inserted into the ear canal