The Collaborative International Dictionary of English v.0.48:

Mesembryanthemum \Me*sem`bry*an"the*mum\, n. [NL., fr. Gr.
   meshmbri`a midday + 'a`nqos flower.] (Bot.)
   A genus of herbaceous or suffruticose plants, chiefly natives
   of South Africa. The leaves are opposite, thick, and fleshy.
   The flowers usually open about midday, whence the name.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Mesembryanthemum
    n 1: South African annual or biennial plants having flowers that
         open only in bright sunlight [syn: Mesembryanthemum,
         genus Mesembryanthemum]