The Collaborative International Dictionary of English v.0.48:

Monocotyledon \Mon`o*cot`y*le"don\, n. [Mono- + cotyledon: cf.
   F. monocotyl['e]done.] (Bot.)
   A plant with only one cotyledon, or seed lobe; a member of
   the Monocotyledonae.
   [1913 Webster]

   Note: The plural, monocotyledons, is used as the name of a
         large class of plants (the Monocotyledones, or
         Monocotyledonae), and is generally understood to be
         equivalent to the term endogens.
         [1913 Webster] Monocotyledonae

WordNet (r) 3.0 (2006):

monocotyledon
    n 1: a monocotyledonous flowering plant; the stem grows by
         deposits on its inside [syn: monocot, monocotyledon,
         liliopsid, endogen]