An international auxiliary language based on the words that are common to the major West-European languages and on a simplified Anglo-Romance grammar. It was first published in 1951 by the International Auxiliary Language Association.

The grammar and vocabulary of Interlingua were initially published in 1951. Alexander Gode, director of IALA during its later years, was one of the prime movers in this effort. He published a survey of the grammar, a one-way dictionary (Interlingua to English), and an introductory book entitled Interlingua a Prime Vista ("Interlingua at First Sight").

The international vocabulary has absorbed materials of the most varied origins, but its center of gravity lies in the sphere of the Greco-Latin tradition. It can be collected within the confines of a homogeneous group of source or control languages which not only represent the Greco-Latin tradition in our time but have likewise absorbed all significant international words radiated from other centers. This group was defined by IALA as the Anglo-Romance group of languages (English, French, Italian, Spanish and Portuguese) with German and Russian as potential contributors.

A word is eligible in Interlingua if it occurs with same meaning and etymology in three of the four main control units. Spanish and Portuguese are regarded together as one control unit. If support is found in two control units, German and Russian are checked to provide the last support. Grammatical words, required to operate the language, are taken from Latin if the regular procedure fails.

The grammar of Interlingua is simplified by discarding grammatical features absent in at least one of the control languages. As a result it is functionally very similar to English grammar, since that is in many respects simpler than the grammar of the Romance languages, German and Russian.

Interlingua is the second-most spoken international auxiliary language after Esperanto, although it claims to be the most widely understood IAL by virtue of the naturality of its grammar and vocabulary, allowing polyglots and especially speakers of Romance languages to read and understand it at first sight. The number of speakers is estimated to be between a thousand and several thousands.

Interlingua has some enthusiastic supporters in North and South America, Europe, Russia, and particularly Scandinavia. There are some Interlingua web pages, and several periodicals, including Panorama in Interlingua from the Union Mundial pro Interlingua and the magazines of the national societies allied with it. There are several active mailing lists, and Interlingua is also in use in Usenet newsgroups, particularly in the europa.* hierarchy.

Every two years the Union Mundial pro Interlingua organizes an international conference in a different European country, which is usually attended by 50-75 people. In the year in between, the Scandinavian Interlingua societies co-organize a conference in Sweden, which welcomes not only Scandinavian but also international visitors.