TITLE: What does $\mathrm{Re}(x)$ mean?
QUESTION [12 upvotes]: 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.

REPLY [15 votes]: 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 $z = \mathrm{Re}(z) + \mathrm{Im}(z)i$.