TITLE: What is this R-like symbol power 2?
QUESTION [7 upvotes]: I found this in a computer medical research text. 
What is the meaning of this R-like letter? S, in this context is an iso-intensity surface.
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Since context is not sufficient, I think it is a good idea to provide the text here:

REPLY [16 votes]: It's hard to tell without a bit more context (and since I don't know what an iso-intensity surface is). But I think it would more commonly be written $\mathbb{R}^2$, which is the set of pairs of real numbers.
So my guess would be that saying $(x,y)\in \Re^2$ just means that $x$ and $y$ are both real numbers.