Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   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.

Source : WordNet®

cotyledon
     n : embryonic leaf in seed-bearing plants [syn: {seed leaf}]