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I see this all the time in Mathematica output as well as in text, such as near the top of the Wikipedia Beta function page.
2 Answers
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If a complex number $z$ is written as $z = a + bi$, then Re$(z) = a$ and Im$(z) = b$. (At risk of stating the obvious, "Re" stands for "Real" and "Im" stands for "Imaginary".)
If we visualize complex numbers as vectors in $\mathbb{R}^2$, Re is the projection onto the real axis, and Im is onto the imaginary axis. So $z = \mathrm{Re}(z) + \mathrm{Im}(z)i$.
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The real part of the complex number x. If you haven't seen complex numbers before, they're a two-dimensional version of the normal real numbers.
