Cumulative distribution function (Why isn't it over x, but some other variable)? 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 $z$.

$z$ there is just a dummy variable. One can use anything other than $x$ there to avoid confusion since one takes the integral from $-\infty$ to $x$.
