exists and is an alternate of. Merge this question into. Split and merge into it. Answer by Dalitton. Confidence votes 32. The letter u is often used to symbolize the greek letter mu, because it resembles it. Mu is used to denote micro-or 10^(-6). So... uM is a commonly used way to denote micrometers, or microns. ! x 10^-6 Meter.