this paper and also wiki here, show CDF as follows (different letters, but same concept):
Given PDF $f_{X}$: $P\{X\in \chi\} = \int_\chi{f_{X}(x) dx} $
Then CDF $F_{X}(x)$ is defined as: $ F_{X}(x)=\int_{-\infty}^{x} f_X (z) dz$
My question is, where is the $z$ coming from? If you look at a plot of CDF, the $x$-axis still shows, well, $x$. Wiki has $t$ instead of $z$.