Language isolate
A language with no demonstrated relatives — a family of one.
Glottolog defines isolates plainly as “one-member families”, and counts 183 of them against 246 families.
The word describes the state of the evidence, not a claim about origins. An isolate is a language whose relationship to any other has not been demonstrated — which may mean it has none, or that its relatives are gone, or that the comparative work has not been done. Glottolog’s caution about thin historical-comparative research applies here with particular force.
Ket, Kusunda and Taushiro are isolates in this Atlas. Each one that falls silent takes a whole branch of human language with it, with nothing left to compare it against.