Entry 1 sense Webster, 1913 Nereidian · Ne·re·id·i·an · IPA /ˌnɛriˈɪdiən/ 01 n. Any annelid resembling Nereis, or of the family Lycoridae or allied families. 1. Any annelid resembling Nereis, or of the family Lycoridae or allied families.(Zool.)