The Collaborative International Dictionary of English v.0.48:

Cellar \Cel"lar\, n. [OE. celer, OF. celier, F. celier, fr. L.
   cellarium a receptacle for food, pantry, fr. cella storeroom.
   See Cell.]
   A room or rooms under a building, and usually below the
   surface of the ground, where provisions and other stores are
   kept.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cellar
    n 1: the lowermost portion of a structure partly or wholly below
         ground level; often used for storage [syn: basement,
         cellar]
    2: an excavation where root vegetables are stored [syn: root
       cellar, cellar]
    3: storage space where wines are stored [syn: cellar, wine
       cellar]