Jeju Island is South Korea's largest island, covering an area of 1,833.2 km2 (707.8 sq mi), which is 1.83 percent of the total area of the country. Alongside outlying islands, it is part of Jeju Province.

Jeju, often called Jejueo or Jejuan in English-language scholarship, is a Koreanic language originally from Jeju Island, South Korea. It is not mutually intelligible with mainland Korean dialects. While it was historically considered a divergent Jeju dialect of the Korean language, it is increasingly referred to as a separate language in its own right. It is declining in usage, and was classified by UNESCO in 2010 as critically endangered, the highest level of language endangerment possible. Revitalization efforts are ongoing.