The Collaborative International Dictionary of English v.0.48:

Noctiluca \Noc`ti*lu"ca\, n.; pl. Noctilucae. [L. noctiluca
   something that shines by night, fr. nox, noctis, night +
   lucere to shine, lux light.]
   1. (Old Chem.) That which shines at night; -- a fanciful name
      for phosphorus.
      [1913 Webster]

   2. (Zool.) A genus of marine flagellate Infusoria, remarkable
      for their unusually large size and complex structure, as
      well as for their phosphorescence. The brilliant diffuse
      phosphorescence of the sea is often due to myriads of
      Noctilucae.
      [1913 Webster]

WordNet (r) 3.0 (2006):

noctiluca
    n 1: large bioluminescent marine protozoan [syn: noctiluca,
         Noctiluca miliaris]