A constructed language (shortened to conlang) is a language whose phonology, grammar, orthography, 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.

By contrast, some philosophers have argued that all human languages are conventional or artificial. François Rabelais’s fictional giant Pantagruel, for instance, said: “It is a misuse of terms to say that we have natural language; languages exist through arbitrary institutions and the conventions of peoples. Voices, as the dialecticians say, don’t signify naturally, but capriciously.”