Interlingua (ɪntərˈlɪŋɡwə; ISO 639 language codes ia, ina) is an international auxiliary language (IAL), developed between 1937 and 1951 by the International Auxiliary Language Association (IALA). It ranks among the top three most widely used IALs (after Esperanto and perhaps Ido), and is the most widely used naturalistic IAL: in other words, its vocabulary, grammar and other characteristics are largely derived from natural languages. Interlingua was developed to combine a simple, mostly regular grammar with a vocabulary common to the widest possible range of languages, making it unusually easy to learn, at least for those whose native languages were sources of Interlingua's vocabulary and grammar. Conversely, it is used as a rapid introduction to many natural languages.
Interlingua literature maintains that (written) Interlingua is comprehensible to the hundreds of millions of people who speak a Romance language, though it is actively spoken by only a few hundred.

The name Interlingua comes from the Latin words inter, meaning between, and lingua, meaning tongue or language. These morphemes are identical in Interlingua. Thus, Interlingua would be "between language", or intermediary language.