How likely a language is to stop being spoken within a generation — based on whether children are still learning it at home, not how many speakers exist.

Threatened — children still learn it, but its future is uncertain.
Shifting — parents speak it but are not passing it on.
Moribund — only grandparents speak it fluently.
Nearly Extinct — a handful of elderly speakers remain.

How thoroughly a language has been recorded by linguists — grammars, dictionaries, audio, written texts.

A language can be nearly extinct but well-documented (linguists got there in time), or have thousands of speakers but almost no written record.

Well documented — a full grammar exists.
Partially documented — grammar sketch or texts only.
Undocumented — wordlist or less. Racing against time.