The Collaborative International Dictionary of English v.0.48:

Contrabass \Con`tra*bass"\, n. (Mus.)
   The lowest stringed instrument of the violin family; the
   double bass; -- also applied to any instrument of the same
   deep range as the stringed double bass; as, the contrabass
   ophicleide; the contrabass tuba or bombardon.
   [1913 Webster + Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

contrabass
    adj 1: pitched an octave below normal bass instrumental or vocal
           range; "contrabass or double-bass clarinet" [syn:
           contrabass, double-bass]
    n 1: largest and lowest member of the violin family [syn: bass
         fiddle, bass viol, bull fiddle, double bass,
         contrabass, string bass]