The Collaborative International Dictionary of English v.0.48:

Cotyledon \Cot`y*le"don\ (k?t`?-l?"d?n), n. [Gr.??? a cupshaped
   hollow, fr. ???. See Cotyle.]
   1. (Anat.) One of the patches of villi found in some forms of
      placenta.
      [1913 Webster]

   2. (Bot.) A leaf borne by the caulicle or radicle of an
      embryo; a seed leaf.
      [1913 Webster]

   Note: Many plants, as the bean and the maple, have two
         cotyledons, the grasses only one, and pines have
         several. In one African plant (Welwitschia) the
         cotyledons are permanent and grow to immense
         proportions.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cotyledon
    n 1: embryonic leaf in seed-bearing plants [syn: cotyledon,
         seed leaf]