The Collaborative International Dictionary of English v.0.48:

Purslane \Purs"lane\, n. [OF. porcelaine, pourcelaine (cf. It.
   porcellana), corrupted fr. L. porcilaca for portulaca.]
   (Bot.)
   An annual plant (Portulaca oleracea), with fleshy,
   succulent, obovate leaves, sometimes used as a pot herb and
   for salads, garnishing, and pickling.
   [1913 Webster]

   Flowering purslane, or Great flowered purslane, the
      Portulaca grandiflora. See Portulaca.

   Purslane tree, a South African shrub (Portulacaria Afra)
      with many small opposite fleshy obovate leaves.

   Sea purslane, a seashore plant (Arenaria peploides) with
      crowded opposite fleshy leaves.

   Water purslane, an aquatic plant (Ludwiqia palustris) but
      slightly resembling purslane.
      [1913 Webster]

WordNet (r) 3.0 (2006):

purslane
    n 1: a plant of the family Portulacaceae having fleshy succulent
         obovate leaves often grown as a potherb or salad herb; a
         weed in some areas