'mu' is a letter in the Greek alphabet. It looks like this: μ
If you need labels for quantities or variables in math or science,
you can use any letter from any alphabet you like ... just be sure
to explain what the symbol stands for somewhere on the page
where the equation or formula is.
I've seen 'mu' used to stand for 'micro-' when writing units.
For example:
" μ-meter " ... one millionth of a meter
" μ-farad " ... one millionth of a farad
" μ-second " ... one millionth of a second