noun
also ep·i·nych·i·um ˌepəˈnikēəm ; or ep·i·o·nych·i·um ˌepēōˈn-
( -s )
Etymology: New Latin, from epi- + -onychium
1. : the thickened layer of epidermal tissue over the developing fetal fingernail or toenail that disappears before birth except over the base of the nail where it persists as the perionychium
2. : the quick of a nail