This seems to be a term introduced by Prof. Vitek in their paper on type stability (linked above), where they write:
That is, they are referring to a function where concrete types of all variables (not just the return value) can be inferred by an ideal compiler (which only knows the argument types), i.e. one where
@code_typed is “green”.
It’s not a new concept in general in the Julia community, but people have talked about in different ways, e.g. as “type stability” (of the variables, not the return value) or as “type inferability”, and Jan was pushing for more precise terminology.