A language is a structured system of communication used by humans. Languages can be based on speech and gesture (spoken language), sign, or writing. The structure of language is its grammar and the free components are its vocabulary. Many languages, including the most widely-spoken ones, have writing systems that enable sounds or signs to be recorded for later reactivation. Human language is unique among the known systems of animal communication in that it is not dependent on a single mode of transmission (sight, sound, etc.), is highly variable between cultures and across time, and affords a much wider range of expression than other systems.
Estimates of the number of human languages in the world vary between 5,000 and 7,000. Precise estimates depend on an arbitrary distinction (dichotomy) being established between languages and dialects
. Natural languages are spoken, signed, or both; however, any language can be encoded into secondary media using auditory, visual, or tactile stimuli – for example, writing, whistling, signing, or braille. In other words, human language is modality-independent, but written or signed language is the way to inscribe or encode the natural human speech or gestures.
Depending on philosophical perspectives regarding the definition of language and meaning, when used as a general concept, “language” may refer to the cognitive ability to learn and use systems of complex communication, or to describe the set of rules that makes up these systems, or the set of utterances that can be produced from those rules. All languages rely on the process of semiosis to relate signs to particular meanings. Oral, manual and tactile languages contain a phonological system that governs how symbols are used to form sequences known as words or morphemes, and a syntactic system that governs how words and morphemes are combined to form phrases and utterances.