The Greek letter µ (mu) is sometimes used as the symbol for "micron", which is a micrometer (one millionth of a meter). I think that the use of "micron" for this has not been officially encouraged for several decades, but it's still in common use.

One also may sometime hear "micron" used as a unit of pressure measurement, as in, "this vacuum pump pulls to 10 microns or better". There, it means "10 micrometers of mercury". That usage is also no longer officially encouraged.