The Collaborative International Dictionary of English v.0.48:

Portulaca \Por`tu*la"ca\, n. [L., purslane.] (Bot.)
   A genus of polypetalous plants; also, any plant of the genus.
   [1913 Webster]

   Note: Portulaca oleracea is the common purslane. Portulaca
         grandiflora is a South American herb, widely
         cultivated for its showy crimson, scarlet, yellow, or
         white, ephemeral blossoms.
         [1913 Webster]

WordNet (r) 3.0 (2006):

portulaca
    n 1: a plant of the genus Portulaca having pink or red or purple
         or white ephemeral flowers