The Collaborative International Dictionary of English v.0.48:

bellbottoms \bellbottoms\ n.
   trousers with legs that flare; commonly worn as part of a
   sailor's uniform; -- such absurdly wide hems were also
   fashionable in the 1960s.

   Syn: bellbottom trousers.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bell-bottoms
    n 1: trousers with legs that flare; worn by sailors; absurdly
         wide hems were fashionable in the 1960s [syn: bellbottom
         trousers, bell-bottoms, bellbottom pants]