Jeju (Jeju: , ; , or , ), often called Jejueo or Jejuan in English-language scholarship, is a Koreanic language traditionally spoken on Jeju Island, South Korea. While often classified as a divergent Jeju dialect (, ) of the Korean language, the variety is referred to as a language in local government and increasingly in both South Korean and foreign academia. Jeju is not mutually intelligible with mainland Korean dialects. The consonants of Jeju are similar to those of Seoul Korean, but Jeju has a larger and more conservative vowel inventory. Jeju is a head-final, agglutinative, suffixing language like Korean. Nouns are followed by particles that may function as case markers. Verbs inflect for tense, aspect, mood, evidentiality, relative social status, formality, and other grammatical information. Korean and Jeju differ significantly in their verbal paradigms. For instance, the continuative aspect marker of Jeju and the mood or aspect distinction of many Jeju connective suffixes are absent in Korean. Most of the Jeju lexicon is Koreanic, and the language preserves many Middle Korean words now lost in Standard Korean. Jeju may also have a Peninsular Japonic substratum, but this argument has been disputed by Lee (2017). Jeju was already divergent from Seoul Korean by the fifteenth century, and was unintelligible to mainland Korean visitors by the sixteenth century. The language was severely undermined by the Jeju uprising of 1948, the Korean War, and the modernization of South Korea. All fluent speakers remaining in Jeju Island are now over seventy years old. Most people in Jeju Island now speak a variety of Korean with a Jeju substratum. The language may be somewhat more vigorous in a diaspora community in Osaka, Japan, as many Jeju people migrated to Osaka in the 1920s, but even there, younger members of the community speak Japanese. Since 2010, UNESCO has designated the language as critically endangered, the highest level of language endangerment possible. Revitalization efforts are ongoing. Nomenclature and relationship to Korean Jeju is closely related to Korean. It was traditionally considered an unusually divergent dialect of Korean, and is still referred to as such by the National Institute of the Korean Language and the South Korean Ministry of Education. While the term "Jeju language" (, ) was first used in 1947, it was not until the mid-1990s that the term gained currency in South Korean academia. While "Jeju dialect" was still the preferred usage throughout the