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]

WordNet (r) 3.0 (2006):

Chinese cabbage
    n 1: plant with an elongated head of broad stalked leaves
         resembling celery; used as a vegetable in east Asia [syn:
         Chinese cabbage, celery cabbage, napa, pe-tsai,
         Brassica rapa pekinensis]
    2: elongated head of crisp celery-like stalks and light green
       leaves [syn: Chinese cabbage, celery cabbage, Chinese
       celery]