The Collaborative International Dictionary of English v.0.48:

Naumachy \Nau"ma*chy\ (n[add]"m[.a]*k[y^]), n. [L. naumachia,
   Gr. naymachi`a; nay^s ship + ma`chh fight, battle, ma`chesqai
   to fight.]
   1. A naval battle; esp., a mock sea fight put on by the
      ancient Romans.
      [1913 Webster]

   2. (Rom. Antiq.) A show or spectacle representing a sea
      fight; also, a place for such exhibitions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

naumachy
    n 1: a naval spectacle; a mock sea battle put on by the ancient
         Romans [syn: naumachy, naumachia]