The Collaborative International Dictionary of English v.0.48:

Liliopsida \Liliopsida\ n.
   A class comprising seed plants that produce an embryo with a
   single cotyledon and parallel-veined leaves. It includes the
   grasses; lilies; palms; and orchids. It is divided into four
   subclasses or superorders: Alismatidae; Arecidae;
   Commelinidae; and Liliidae.

   Syn: Monocotyledones, class Monocotyledones, Monocotyledonae,
        class Monocotyledonae, class Liliopsida.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Liliopsida
    n 1: comprising seed plants that produce an embryo with a single
         cotyledon and parallel-veined leaves: includes grasses and
         lilies and palms and orchids; divided into four subclasses
         or superorders: Alismatidae; Arecidae; Commelinidae; and
         Liliidae [syn: Monocotyledones, class Monocotyledones,
         Monocotyledonae, class Monocotyledonae, Liliopsida,
         class Liliopsida]