Very often in the literature of research papers and other articles, and maybe text books, on topics of quantum field theory, a theory may be described as a 3+1 or 0+1, or maybe even 1+1 theory. I have never read any definition of this notation but I believe that the general form is (d-1,1) where d is the overall spacetime dimension. Thus, a 4-dimension space time theory might be described as 3+1; or, sometimes (3,1).
A few examples I know of are that the motion of a classical particle that is parameterized by time would simply be a 0+1 theory. Or, I have read that a particular String Theory is described as a 1+1 theory.
I have just exhausted any knowledge I might have gleaned on this question though. I have never seen any formal definition or motivation for its use.
My question, who and when was this notation developed and what was the motivation behind it. In the very simple examples I have seen, it does not seem to add much than what a few words could say more clearly (at least to a philistine such as I).