Permanent Notes

Permanent Notes are a concept in Zettelkasten. David Kadavy has a great description in Digital Zettelkasten:

Permanent notes are explanations of a single idea, annotated with metadata about the subject of the note, other notes that note is related to, and the source of the note.

For me, I use the topic tag, and Topic Notes.

Andy Matuschak calls these Evergreen Notes which is also a great name for them.