The Collaborative International Dictionary of English v.0.48:

Kohl-rabi \Kohl"-ra`bi\, n.; pl. Kohl-rabies. [G. Cf. Cole,
   Rape the plant.] (Bot.)
   A variety of cabbage, in which the edible part is a large,
   turnip-shaped swelling of the stem, above the surface of the
   ground.
   [1913 Webster]

WordNet (r) 3.0 (2006):

kohlrabi
    n 1: plant cultivated for its enlarged fleshy turnip-shaped
         edible stem [syn: kohlrabi, Brassica oleracea
         gongylodes]
    2: fleshy turnip-shaped edible stem of the kohlrabi plant [syn:
       kohlrabi, turnip cabbage]