Peninsular Japonic in the context of "Japonic languages"

Jeju (Jeju: 제줏말; Jeju RR: Jejunmal, or Korean: 제주어; RR: Jejueo, or 제주말; Jejumal), 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.

The consonants of Jeju are similar to those of the Seoul dialect of 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 features and words now lost in Standard Korean. Jeju may also have a Peninsular Japonic substratum, but this argument has been disputed.