relief (n): the projection of a figure or part from the ground or plane on which it is formed, as in sculpture or similar work.
bas relief (n): relief sculpture in which the figures project slightly from the background.
If you are an artist or archeologist, then the differences between “bas relief,” “relief,” and “high relief” (aka “alto-relievo”) are significant.
If you aren’t an an artist or archeologist, those differences aren’t significant, and you probably just need to use the term “relief.” Especially if you’re describing something for the benefit of fantasy adventurers, as most of them couldn’t tell the difference. Or consider using the word “carving” or “sculpture.”
I think a lot of gamers today use “bas relief” when they just mean “relief” because Gary Gygax used it in some 1e adventures. I’m pretty sure Gygax knew the difference, but you and I and the other 10-year-old kids read the phrase and interpreted it to mean “a sculpture-carving on the wall” rather than “a type of sculpture-carving on the wall where the figures project 50% or less from the background.”
(Filing this under grammar, just so I don’t have to create another category for “word issues.”)