U+068B Arabic Letter Dal with Dot Below and Small Tah

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A constructed language (shortened to conlang) is a language whose phonology, grammar, and vocabulary, instead of having developed naturally, are consciously devised for some purpose, which may include being devised for a work of fiction. A constructed language may also be referred to as an artificial, planned or invented language, or (in some cases) a fictional language. Planned languages (or engineered languages/engelangs) are languages that have been purposefully designed; they are the result of deliberate, controlling intervention and are thus of a form of language planning.

There are many possible reasons to create a constructed language, such as to ease human communication (see international auxiliary language and code); to give fiction or an associated constructed setting an added layer of realism; for experimentation in the fields of linguistics, cognitive science, and machine learning; for artistic creation; for fantasy role-playing games; and for language games. Some people may also make constructed languages as a hobby.

The expression planned language is sometimes used to indicate international auxiliary languages and other languages designed for actual use in human communication. Some prefer it to the adjective artificial, as this term may be perceived as pejorative. Outside Esperanto culture, the term language planning means the prescriptions given to a natural language to standardize it; in this regard, even a "natural language" may be artificial in some respects, meaning some of its words have been crafted by conscious decision. Prescriptive grammars, which date to ancient times for classical languages such as Latin and Sanskrit, are rule-based codifications of natural languages, such codifications being a middle ground between naïve natural selection and development of language and its explicit construction. The term glossopoeia is also used to mean language construction, particularly construction of artistic languages.

Conlang speakers are rare. For example, the Hungarian census of 2011 found 8,397 speakers of Esperanto, and the census of 2001 found 10 of Romanid, two each of Interlingua and Ido and one each of Idiom Neutral and Mundolinco. The Russian census of 2010 found that in Russia there were about 992 speakers of Esperanto (on place 120) and nine of the Esperantido Ido.