An international auxiliary language (sometimes acronymized as IAL or contracted as auxlang) is a language meant for communication between people from all 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 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.
Languages of dominant societies over the centuries have served as lingua francas that have sometimes approached the international level. Latin, Greek, Sanskrit, Persian and the Mediterranean Lingua Franca were used in the past. In recent times, Standard Arabic, Standard Chinese, English, French, German, Italian, Portuguese, Russian and Spanish have been used as such in many parts of the world. However, as lingua francas are traditionally associated with the very dominance—cultural, political, and economic—that made them popular, they are often also met with resistance. For this and other reasons, some have turned to the idea of promoting a constructed language as a possible solution, by way of an "auxiliary" language, one example of which being Esperanto.
The use of an intermediary auxiliary language (also called a "working language", "bridge language", "vehicular language" or "unifying language") to make communication possible between people not sharing a first language, in particular when it is a third language, distinct from both mother tongues, may be almost as old as language itself.