The Collaborative International Dictionary of English v.0.48:

Sofa \So"fa\, n.; pl. Sofas. [Ar. soffah, from saffa to
   dispose in order: cf. F. sofa, It. sof[`a].]
   A long seat, usually with a cushioned bottom, back, and ends;
   -- much used as a comfortable piece of furniture.
   [1913 Webster]

         Let fall the curtains, wheel the sofa round. --Cowper.
   [1913 Webster]

   Sofa bed, a sofa so contrived that it may be extended to
      form a bed; -- called also sofa bedstead.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sofa
    n 1: an upholstered seat for more than one person [syn: sofa,
         couch, lounge]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "sofa":
   bed, bedstead, bunk, chesterfield, couch, davenport, day bed,
   divan, doss, gurney, kip, litter, lounge, love seat, ottoman,
   settee, settle, spoonholder, squab, stretcher, studio couch,
   tete-a-tete, the hay, the sack