$u and $v are also standard for coordinates, as of course is $r (in the company of a $theta).

And plenty of other things use single-letter variables by convention. If I were implementing a quadratic equation solver, for example, I would consider it obfuscation to name the coefficients anything other than $a, $b, and $c.

The question should never be "does this variable name meet some arbitrary standard", but rather "will my successor, trying to maintain this code in a few years' time, immediately grasp what this variable is for". If the answer is "yes" (and can be justified!), then any name is potentially appropriate, regardless of length.