My admittedly arbitrary metric:
There are records/artists that some members of the "general public" are aware of; Miles Kind of Blue, Coltrane Love Supreme, Bill Evans Sunday at Village Vanguard, Louis Armstrong Hot Fives and Sevens, etc. These great records are not underrated.
On the other hand, unless you have a good background in jazz, you wouldn't know who Oliver Nelson is; hence, underrated. Does this make sense?

