sign language, gestural communication used as an alternative or replacement for speech. Sign languages resemble oral languages in every way other than their modality. As with oral languages, sign languages are acquired spontaneously and have highly intricate, rule-governed grammar and phonology. The three classes of features that make up individual signs are hand configuration, movement, and position to the body. Sign languages include those of Trappist monks, who have a rule of silence, and Plains Indians, where speakers of mutually unintelligible languages communicated freely. Australian aborigines and people of Sudan and the Sahara also have a complete sign language. Many languages have conventionalized body gestures elaborated to accompany or supplement speech, e.g., the Neapolitan gesture language.

The widely used manual language of the deaf, or language of signs, was first systematized in the 18th cent. by the French abbé Charles Michel de l'Épée. It was brought to the United States by T. H. Gallaudet. As with any sign language, only a small percentage of signs suggest the form of thought they represent. Such sign languages also may have a syntax and grammar that differs dramatically from the language spoken locally. This is true, for instance, of American Sign Language, which, developed for the deaf, is a non-English system used in the United States and parts of Canada. A number of written systems for representing manual languages have been developed, and dictionaries of signs have been compiled. Often sign language is taught along with speechreading (see lip reading) and with a manual alphabet, i.e., a method of forming the letters of the alphabet by fixed positions of the fingers in the air. See also deafness.

Any means of communication through bodily movements, especially of the hands and arms, rather than through speech. It has long been used by speakers of mutually unintelligible languages—for example, various Plains Indian tribes in 19th-century North America communicated via a sign language—and is widely used for communication by the deaf. Charles-Michel, abbé de l'Épée (1712–89), developed the first sign language for the deaf in the mid-18th century; his system developed into French Sign Language (FSL), still used in France. Transported to the U.S. in 1816 by Thomas Gallaudet (1787–1851), it evolved into American Sign Language (ASL, or Ameslan), now used by more than half a million people. These and other national sign languages generally express concepts rather than elements of words and thus have more in common with each other than with their countries' spoken languages.

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