I dug out one of my old textbooks for this:
"A language is a system of relatively arbitrary symbols and grammatical signals that change across time and that members of a community share and use for several purposes: to interact with each other, to communicate their ideas, emotions, and intentions, and to transmit their culture from generation to generation."
p. 31
American Sign Language, a teacher's resource text on grammar and culture
by Charlotte Baker and Dennis Cokely
1980 T.J. Publishers, Inc., Silver Spring, MD
("Green Book" series)
The authors use the entire chapter to break down the various components of their definition of language, and prove that ASL meets all the requirements.
The rest of the book has chapters on ASL signs formation and variations (region, racial/ethnic group, sex, age, context), nouns and verbs, compounds and contractions, fingerspelled loan signs, idioms, sentence types, time, pronominalization, subjects and objects, sign order and topicalization, body and gaze shifting, classifiers, pluralization, locatives, temporal aspect, inflections for distributional aspect, and other topics.
