The Collaborative International Dictionary of English v.0.48:

Ovum \O"vum\ ([=o]"v[u^]m), n.; pl. L. Ova ([=o]"v[.a]), E.
   Ovums ([=o]"v[u^]mz). [L., an egg. See Oval.]
   1. (Biol.) A more or less spherical and transparent cell,
      which by a process of multiplication and growth develops
      into a mass of cells, constituting a new individual like
      the parent; an egg, spore, germ, or germ cell. See Illust.
      of Mycropyle.
      [1913 Webster]

   Note: The ovum is a typical cell, with a cell wall, cell
         substance, nucleus, and nucleolus. In man and the
         higher animals the cell wall, a vertically striated
         membrane, is called the zona pellucida; the cell
         contents, the vitellus; the nucleus, the germinal
         vesicle; and the nucleolus, the germinal spot. The
         diameter of the ripe ovum in man and the domestic
         animals varies between 1-200 and 1-120 of an inch.
         [1913 Webster]

   2. (Arch.) One of the series of egg-shaped ornaments into
      which the ovolo is often carved. --Gwilt.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ovum
    n 1: the female reproductive cell; the female gamete [syn:
         ovum, egg cell]