The Collaborative International Dictionary of English v.0.48:

sign language \sign" lang`uage\ (s[imac]n" l[a^]ng`w[asl]j), n.
   A form of language for communicating by use of gestures made
   by the hands, rather than by speech. It includes alphabets
   made by hand gestures, as well as proper languages formed
   from signs. Among the latter is the American Sign Language
   (ASL), used by the deaf. See also dactylology.
   [PJC]

WordNet (r) 3.0 (2006):

sign language
    n 1: language expressed by visible hand gestures [syn: sign
         language, signing]