Sign languages (also known as signed languages ) are languages that
use the visual-manual modality to convey meaning, instead of spoken words.
Sign languages are expressed through manual articulation in combination with
non-manual markers. Sign languages are full-fledged natural languages with
their own grammar and lexicon. Sign languages are not universal and are
usually not mutually intelligible, although there are also similarities among
different sign languages.
Linguists consider both spoken and signed communication to be types of natural
language, meaning that both emerged through an abstract, protracted aging
process and evolved over time without meticulous planning.