Cornish is a Southwestern Brittonic language of the Celtic language family. Along with Welsh and Breton, Cornish is descended from the Common Brittonic language spoken throughout much of Great Britain before the English language came to dominate. For centuries, until it was pushed westwards by English, it was the main language of Cornwall, maintaining close links with its sister language Breton, with which it was mutually intelligible, perhaps even as long as Cornish continued to be spoken as a vernacular. Cornish continued to function as a common community language in parts of Cornwall until the mid 18th century, and there is some evidence for traditional speakers of the language persisting into the 19th century.

An endangered language or moribund language is a language that is at risk of disappearing as its speakers die out or shift to speaking other languages. Language loss occurs when the language has no more native speakers and becomes a "dead language". If no one can speak the language at all, it becomes an "extinct language". A dead language may still be studied through recordings or writings, but it is still dead or extinct unless there are fluent speakers. Although languages have always become extinct throughout human history, they are currently dying at an accelerated rate because of globalization, mass migration, cultural replacement, imperialism, neocolonialism and linguicide.