The Collaborative International Dictionary of English v.0.48:

pak-choi \pak-choi\ n.
   An Asiatic plant (Brassica rapa chinensis) grown for its
   cluster of edible white stalks with dark green leaves.

   Syn: bok choy, bok choi, pak choi, Chinese white cabbage,
        Brassica rapa chinensis.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

pe-tsai \pe-tsai\ n.
   A plant with an elongated celerylike head of broadstalked
   leaves used as a vegetable in east Asia.

   Syn: Chinese cabbage, celery cabbage, napa, Brassica rapa
        pekinensis.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

napa \napa\ n.
   A plant (Brassica rapa pekinensis) with an elongated
   celerylike head of broadstalked leaves used as a vegetable in
   east Asia; called also Chinese cabbage.

   Syn: Chinese cabbage, celery cabbage, pe-tsai, Brassica
        rapa pekinensis.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Brassica \Bras"si*ca\, prop. n. [L., cabbage.] (Bot.)
   A genus of plants embracing several species and varieties
   differing much in appearance and qualities: such as the
   common cabbage (Brassica oleracea), broccoli, cauliflowers,
   etc.; the wild turnip (Brassica campestris); the common
   turnip (Brassica rapa); the rape or coleseed (Brassica
   napus), etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Brassica rapa
    n 1: widely cultivated plant having a large fleshy edible white
         or yellow root [syn: turnip, white turnip, Brassica
         rapa]