An endangered language is a language that is at risk of falling out of use, generally because it has few surviving speakers. If it loses all of its native speakers, it becomes an extinct language. A language may be endangered in one area but show signs of revitalisation in another, as with the Irish language.
UNESCO defines four levels of language endangerment between "safe" (not endangered) and "extinct":
References
List of endangered languages in Europe Wikipedia(Text) CC BY-SA