An endangered language is a language that is at risk of falling out of use. If it loses all its native speakers, it becomes a dead language. If eventually no one speaks the language at all it becomes an extinct language. It is believed that 90% of the approximately 7,000 languages currently spoken in the world will have become extinct by 2050, as the world's language system has reached a crisis and is dramatically restructuring.
While there is no definite threshold for identifying a language as endangered, three main criteria are used as guidelines: 1) The number of speakers currently living; 2) the mean age of native and/or fluent speakers; and 2) the percentage of the youngest generation acquiring fluency with the language in question. Once a language is determined to be endangered, there are two basic steps that need to be taken in order to stabilize or rescue the language. Language documentation is the process by which the language is documented in terms of its grammar, its lexicon, and its oral traditions (e.g. stories, songs, religious texts). Language revitalization is the process by which a language community through political, community, and educational means attempts to increase the number of active speakers of the endangered language.
Normally the transition from a dead to an extinct language occurs when a language undergoes language death while being directly replaced by a different one. For example, Native American languages were replaced by English, French, Portuguese, or Spanish as a result of colonization. The Coptic language, replaced by Arabic in its native Egypt, was once thought to be extinct. Language extinction may also occur when a language evolves into a new language or family of languages. An example of this was Old English, a forerunner of Modern English.
Estimates of future language loss range from half of more than 6000 currently spoken languages being lost in the next 200 years, to 90% by the year 2050. Languages – as not simply bodies of vocabulary or sets of grammatical rules, but "old growth forests of the mind" – for the many and unique cultures of the world reflect different ways of being, thinking, and knowing. All of that is lost when a language dies out.
Adapted from Wikipedia.