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]

WordNet (r) 3.0 (2006):

pakchoi
    n 1: Asiatic plant grown for its cluster of edible white stalks
         with dark green leaves [syn: bok choy, bok choi,
         pakchoi, pak choi, Chinese white cabbage, Brassica
         rapa chinensis]