The term "rare languages" is sometimes used in newspaper headings, blog posts, etc. For example:
- This Innovative Platform Ensures That Kids in India Have Access to Stories in Their Mother Tongue, The Better India, 26 Jul 2016 (examples: Sanskrit, Banjari, Lambadi, Kuruk, Oraon, Gondi, Mundari, Sadri, Santhali and Kora),
- Preserve rare languages to spread benefits of multilingualism, says expert, The Guardian, 15 Feb 2016 (examples: Sardinian and Scottish Gaelic),
- Economic success 'drives language extinction', BBC News, 3 Sep 2014 (examples: Upper Tanana, Ume Sami, Auvergnat, Bahing),
- For Rare Languages, Social Media Provide New Hope, NPR, 26 Jul 2014 (examples: Aymara, Tongva, Anishinaabemowin),
- About 2,000 rare languages may disappear on Earth in 100 years, PravdaReport, 29 Nov 2007 (examples: Siletz Dee-ni, which had only one speaker at the time),
- Top Five Rarest Languages, Network Languages Ltd, 10 Mar 2014 (Njerep, Chemehuevi, Kaixana or Caixana, Taushiro or Pinche, Liki).
When you actually read the articles, they are usually about endangered languages. So my question is: Do linguists use a definition of rare language that is different from "endangered language"? (That is, if they actually use the term "rare language".)
PS: There appears to be no agreed-upon definition of endangered or threatened language. The abstract of this article says that
(...) languages have been classified as threatened if the number of speakers is less than 100, 500, 1,000, 10,000, 20,000 or 100,000 (...).
UNESCO uses nine factors to determine whether a language is endangered.
Update: Not all "rare" languages are endangered. In a PBS documentary, the poet Bob Holman says that he visited one of the Goulburn Islands, where there were 400 people and 10 different languages. In spite of a small number of speakers, some of the rare languages in Australia are stable because people don't find it a big deal to learn other languages, and learning the language of another people is a sign of respect.