Interlingue ([interˈliŋɡwe]; ISO 639 ie, ile), originally Occidental ([oktsidenˈtaːl]), is an international auxiliary language created in 1922 and renamed in 1949. Its creator, Edgar de Wahl, sought to achieve maximal grammatical regularity and natural character. The vocabulary is based on pre-existing words from various languages and a derivational system which uses recognized prefixes and suffixes.

Many of Interlingue's derived word forms reflect those common to certain Western European languages, primarily the Romance languages, along with some Germanic vocabulary. Many of its words are formed using de Wahl's rule, a set of rules for regular conversion of all but six verb infinitives into derived words including from Latin double-stem verbs (e.g. vider to see and its derivative vision). The result is a naturalistic and regular language that is easy to understand at first sight for individuals acquainted with certain Western European languages. Readability and simplified grammar, along with the regular appearance of the magazine Cosmoglotta, made Occidental popular in Europe during the years before World War II despite efforts by the Nazis to suppress international auxiliary languages.

Occidental survived the war, but the community had been out of touch with the language's creator since 1939. A Baltic German naval officer and teacher from Estonia, de Wahl refused to leave his Tallinn home for Germany, even after his house was destroyed in the 1943 air raids on the city forcing him to take refuge in a psychiatric hospital. Since most of his mail had been intercepted, he died in 1948 largely unaware of developments in the language. The name change to Interlingue took place the following year for two reasons: (1) to demonstrate to the Soviet Union the language's neutrality, and (2) the expectation of a possible union or closer collaboration with the community around Interlingua, a competing naturalistic project under development. Many users were lost following the latter's appearance in 1951, beginning a period of decline until the advent of the Internet.