The Collaborative International Dictionary of English v.0.48:

Nereid \Ne"re*id\, n.; pl. E. Nereids, L. Nereides. [L.
   Nereis, -idis, Gr. Nhrei:`s Nhrhi:`s, Nhrhi:`dos, a daughter
   of Nereus, a nymph of the sea, fr. Nhrey`s Nereus, an ancient
   sea god; akin to nhro`s wet, Skr. n[=a]ra water, cf. Gr.
   na`ein to flow.]
   1. (Class. Myth.) A sea nymph, one of the daughters of
      Nereus, who were attendants upon Neptune, and were
      represented as riding on sea horses, sometimes with the
      human form entire, and sometimes with the tail of a fish.
      [1913 Webster]

   2. (Zool.) Any species of Nereis. The word is sometimes
      used for similar annelids of other families.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Nereid
    n 1: (Greek mythology) any of the 50 sea nymphs who were
         daughters of the sea god Nereus