The Collaborative International Dictionary of English v.0.48:

Violoncello \Vi`o*lon*cel"lo\ (?; 277), n. [It. violoncello,
   dim. of violone a bass viol. See Violone.] (Mus.)
   A stringed instrument of music; a bass viol of four strings,
   or a bass violin with long, large strings, giving sounds an
   octave lower than the viola, or tenor or alto violin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

violoncello
    n 1: a large stringed instrument; seated player holds it upright
         while playing [syn: cello, violoncello]