How about ![$M\_n(\mathbb{R})^+](http://latex.mathoverflow.net/png?%24M%5Fn%28%5Cmathbb%7BR%7D%29%5E%2B)?  I have seen ![S^+](http://latex.mathoverflow.net/png?S%5E%2B) or ![S\_+](http://latex.mathoverflow.net/png?S%5F%2B) used to denote the set of positive linear transformations in a set ![S](http://latex.mathoverflow.net/png?S) of linear transformations on an inner product space, but this was in the context of operator algebras.