The Collaborative International Dictionary of English v.0.48:

Pronucleus \Pro*nu"cle*us\, n.; pl. Pronuclei (-[imac]). [NL.
   See Pro-, and Nucleus.] (Biol.)
   One of the two bodies or nuclei (called male and female
   pronuclei) which unite to form the first segmentation nucleus
   of an impregnated ovum.
   [1913 Webster]

   Note: In the maturing of the ovum preparatory to
         impregnation, a part of the germinal vesicle (see
         Polar body, under Polar) becomes converted into a
         number of small vesicles, which aggregate themselves
         into a single clear nucleus. which travels towards the
         center of the egg and is called the female pronucleus.
         In impregnation, the spermatozoon which enters the egg
         soon loses its tail, while the head forms a nucleus,
         called the male pronucleus, which gradually travels
         towards the female pronucleus and eventually fuses with
         it, forming the first segmentation nucleus.
         [1913 Webster]

WordNet (r) 3.0 (2006):

pronucleus
    n 1: the nucleus of the ovum or sperm after fertilization but
         before they fuse to form the nucleus of the zygote