Human language is unique because it has the properties of productivity, recursivity, and displacement, and because it relies entirely on social convention and learning. Oral and sign 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. All languages rely on the process of semiosis to relate signs with particular meanings. 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. This is because human language is modality-independent. Natural languages are spoken or signed, but any language can be encoded into secondary media using auditory, visual, or tactile stimuli, for example, in graphic writing, braille, or whistling.
However, estimates vary between 6,000 and 7,000 languages in number. Any estimate of the precise number of languages in the world depends on a partly arbitrary distinction between languages and dialects. The scientific study of language is called linguistics. Language is the human capacity for acquiring and using complex systems of communication, and a language is any specific example of such a system. Braille writing represents language in a tactile form.