Intergenerational transmission
Whether children are still learning the language at home — the single most telling measure of its vitality.
UNESCO’s expert group calls this “the most commonly used factor in evaluating the vitality of a language”: whether or not it is being passed from one generation to the next. Every other label on a profile follows from it.
It is why speaker counts alone mislead. A language with a million speakers and no children learning it is further along the path than one with a thousand who are raising children in it. The count measures the present; transmission measures whether there is a future.
The six degrees UNESCO distinguishes — safe, unsafe, definitively endangered, severely endangered, critically endangered, extinct — are all defined in terms of which generation still speaks, and to whom.