The Collaborative International Dictionary of English v.0.48:

Gametophyte \Ga*me"to*phyte\, n. [Gamete + Gr. fyto`n plant.]
   (Bot.)
   In the alternation of generations in plants, that generation
   or phase which bears sex organs and produces gametes. In the
   lower plants, as the alg[ae], the gametophyte is the
   conspicuous part of the plant body; in mosses it is the
   so-called moss plant; in ferns it is reduced to a small,
   early perishing body; and in seed plants it is usually
   microscopic or rudimentary.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gametophyte
    n 1: the gamete-bearing individual or phase in the life cycle of
         a plant having alternation of generations