The Collaborative International Dictionary of English v.0.48:

celtuce \celtuce\ n.
   1. a type of lettuce (Lactuca sativa asparagina) valued
      especially for its edible stems.

   Syn: stem lettuce, Lactuca sativa asparagina.
        [WordNet 1.5]

   2. a salad green having characteristics of both lettuce and
      celery; stems eaten either raw or cooked.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

celtuce
    n 1: lettuce valued especially for its edible stems [syn:
         celtuce, stem lettuce, Lactuca sativa asparagina]
    2: leaves having celery-like stems eaten raw or cooked