Noun

1. One of several natural languages, typically used by the deaf, where the words and phrases consist of hand shapes, motions, positions, and facial expressions.
   • 2000: Wendy Sandler and Diane Lillo-Martin, in The Handbook of Linguistics (edited by Mark Aronoff and Janie Rees-Miller)

     It is safe to say that the academic world is now convinced that sign languages are real languages in every sense of the term.

2. The sign language (sense 1) that is used locally or that is mistakenly believed to be the only one.

   • I'm taking night classes to learn sign language.

3. Sign languages (sense 1) considered collectively.

   • There are some unique properties found in sign language compared to spoken language.

4. Communication through gestures used when speech is impossible, for example, between monks under a vow of silence or people speaking different languages.
   • 1847: Francis Parkman, The Oregon Trail

     Even Maxwell the trader, who has been most among them, is compelled to resort to the curious sign language common to most of the prairie tribes.