I like the definition of evergreen notes suggested by [[Andy Matuschak]]:
Evergreen notes are written and organized to evolve, contribute, and accumulate over time, across projects. This is an unusual way to think about writing notes: Most people take only transient notes. ... When done well, these notes can be quite valuable: Evergreen note-writing as fundamental unit of knowledge work.
Evergreen is somewhat the opposite from "wrote once and forgot".
To be evergreen it should be revisited, improved and connected to new contexts. This, in turn, requires the note to be atomic and be organized in a way to be able to link to new ideas and contexts.