Language is A human system of communication that uses arbitrary signals, such as voice sounds, gestures, or written symbols. (http://grammar.about.com/od/il/g/languageterm.htm )
"Language is a purely human and non-instinctive method of communicating ideas, emotions and desires by means of voluntarily produced symbols."
(Edward Sapir, Language: An Introduction to the Study of Speech. Harcourt, Brace and Company, 1921)
"A language is a system of arbitrary vocal symbols by means of which a social group cooperates."
(B. Bloch and G. Trager, Outline of Linguistic Analysis. Waverly Press, 1942)
"Language is the mother of thought, not its handmaiden.