epineurium ep·i·neu·ri·um (ěp'ə-n&oobreve;r'ē-əm, -ny&oobreve;r'-) n. pl. ep·i·neu·ri·a (-n&oobreve;r'ē-ə, -ny&oobreve;r'-) The thick sheath of connective tissue surrounding a nerve trunk.