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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.

[edit] Since context is not sufficient, I think it is a good idea to provide the text here:

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    Characters is not what you think it is.2011-06-27
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    Asaf's comment is a reference to the fact (can't really blame the OP) that this question was originally tagged ["characters"](http://math.stackexchange.com/questions/tagged/characters). :-)2011-06-27

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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.

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    This answer is almost certainly correct. I have unfortunately seen this use of this symbol a lot in computing literature. I guess it stems from the fact that it is called \real in LaTeX, presumably because it was meant to represent the operator that gives the real part of a complex number.2011-06-27
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    At the risk of stating what may already be obvious, I would wager that the context is medical imaging (MRI, tomography or something like that), in which case an iso-intensity surface would be the set of points having the same intensity.2011-06-27
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    I have edited the question, adding more context.2011-06-28
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    I had a German professor who used this symbol instead of $\mathbb{R}$... I'm positive it's the same as a real number.2011-06-28