Interlinguistics, also known as cosmoglottics, as the science of planned languages has existed for more than a century. Formalised by Otto Jespersen in 1931 as the science of interlanguages, in more recent times, the field has been more focused with language planning, the collection of strategies to deliberately influence the structure and function of a living language. In this framework, interlanguages become a subset of planned languages, i.e. extreme cases of language planning.

A constructed language 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 a fictional language. Planned languages are languages that have been purposefully designed; they are the result of deliberate, controlling intervention and are thus of a form of language planning.