From 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]
From WordNet (r) 3.0 (2006) :
cotyledon
n 1: embryonic leaf in seed-bearing plants [syn: cotyledon,
seed leaf]