a) One of several natural languages, typically used by the deaf, where the words and phrases consist of hand shapes, motions, positions, and facial expressions.
It is safe to say that the academic world is now convinced that sign languages are real languages in every sense of the term.
b) The sign language (sense 1) that is used locally or that is mistakenly believed to be the only one.
Im taking night classes to learn sign language.
Wikipedia foundation.