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.”)
Sean K Reynolds 
I’ve given this particular matter a lot of thought over the years. As someone who USED TO know the differences between different types of “relief” off the top of my head (but now has to think about it a minute or two before the definitions bubble up from memory) … and as someone who writes and edits for people who generally don’t know the difference (and really don’t care), it’s been a contentious one even in my own mind.
People tend to prefer the term bas relief even though they don’t know the detailed classification for that style of sculpture. For them, “bas relief” is the generic term. For people who DO know the difference, it sounds silly to specify what type of relief when it really doesn’t matter.
I’m not sure I’ve ever come to a firm conclusion on my own. But at times I’ve preferred, as you suggest, to stick to just “relief” unless the type is important. At other times, I’ve thought that since the average person reader seems to prefer “bas relief” and since the type of relief was immaterial, it was best to give the reader what he or she was likely expecting.
…the average person reader…
Note to self: Re-read comments before clicking “post.”
I could suggest filing it under semantics, but that would be…
My father is a bronze sculpture, so that’s how the terms entered my vocabulary at a young age.
That is what makes you special. ;)
And btw I am so tempted to make “baby bronze robot” jokes because of your sculpture/sculptor typo, but I made and corrected so many of my own in the OP that I won’t succumb to temptation. :)
I’ve always used “bas relief” simply on the grounds of every player I’ve ever roleplayed with knew what I was referring to.
Imagine you had a new food for the first time. You ask someone what it is, and they tell you it’s called “cheese-pizza.” Thereafter, whenever you have this food (which is rarely), you call it cheese-pizza. Then you read on Sean’s blog that what you’ve actually been eating is pizza, and in particular you’ve been having cheese pizza but there are other kinds of pizza. Would you still go on calling it cheese-pizza, even when it had ham or sausage or vegetables or pepperoni on it?
Its one of those swings and roundabouts type issues.
For myself, saying bas relief is a better description for the masses than just relief. I’ll try just “relief” in my next D&D session and see.
And if it has an ovine motif, it is a baa relief.
/slap
Or if its a fish, is it a bass relief?