Ido is a constructed language derived from Reformed Esperanto, and similarly designed with the goal of being a universal second language for people of diverse backgrounds. To function as an effective international auxiliary language, Ido was specifically designed to be grammatically, orthographically, and lexicographically regular. It is the most successful of the many Esperanto derivatives, called Esperantidoj.

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.