Ido in the context of Louis Couturat

An international auxiliary language (sometimes acronymized as IAL or contracted as auxlang) is a language meant for communication between people from different nations who do not share a common first language. An auxiliary language is primarily a foreign language and often a constructed language. The concept is related to but separate from the idea of a lingua franca (or dominant language) that people must use to communicate. The study of international auxiliary languages is interlinguistics.

The term "auxiliary" implies that it is intended to be an additional language for communication between the people of the world, rather than to replace their native languages. Often, the term is used specifically to refer to planned or constructed languages proposed to ease international communication, such as Esperanto, Ido and Interlingua. It usually takes words from widely spoken languages. However, it can also refer to the concept of such a language being determined by international consensus, including even a standardized natural language (e.g., International English), and has also been connected to the project of constructing a universal language.

In 1894, L. L. Zamenhof, who was the original creator of the constructed language Esperanto, proposed a complete series of reforms to Esperanto (in other words, an Esperantido). It is notable as the only complete Esperantido to be authored by Zamenhof himself, and it was presented in response to various reforms that had been proposed by others since the language's publication in 1887. The project was eventually rejected by the language's speakers, and subsequently even by Zamenhof himself. Some of the proposed reforms were used in another constructed language, Ido, beginning in 1907.

Novial is an international auxiliary language (IAL) created by Danish linguist Otto Jespersen in 1928. It was designed to facilitate communication between speakers of different native languages. The name of the language is a blend of the Novial word novi (meaning 'new") and IAL.

Jespersen had been an early supporter of another international auxiliary language, Ido, a reformed version of Esperanto, before leaving to create his own language in 1928.