1.  earplug(n = noun.artifact) - an earphone that is inserted into the ear canal;
is a kind of earphone, earpiece, headphone, phone

2.  earplug(n = noun.artifact) - a plug of cotton, wax, or rubber that is fitted into the ear canal for protection against the entry of water or loud noise;
is a kind of plug, stopper, stopple

earplug, n. a piece of wax etc. placed in the ear to protect against cold air, water, or noise.