celeriac

from WordNet (r) 3.0 (2006)

celeriac
    n 1: grown for its thickened edible aromatic root [syn:
         {celeriac}, {celery root}, {knob celery}, {root celery},
         {turnip-rooted celery}, {Apium graveolens rapaceum}]
    2: thickened edible aromatic root of a variety of celery plant
       [syn: {celeriac}, {celery root}]

from The Collaborative International Dictionary of English v.0.48

Celeriac \Ce*le"ri*ac\, n. (Bot.)
   Turnip-rooted celery, a from of celery with a large globular
   root, which is used for food.
   [1913 Webster]