Still, for better or for worse, it is the accepted lingo in the fields of probability & statistics.

As example a quote from http://www.columbia.edu/~ww2040/4106S11/lec0125.pdf which opens with

"What is a random variable?

A (real-valued) random variable, often denoted by X (or some other capital letter), is a

function mapping a probability space (S, P) into the real line R"

Please also note that the term used is "random variable", not "variable".