is unit of length equal to one-millionth (1 x 10^-6) meaning
1 / 1,000,000 of a meter (i.e. micrometer).  The abbreviation is the Greek letter mu (μ) resulting in (μm).  The micrometer is a common unit of measurement for wavelengths of infrared radiation. Typically used in microscopy.

Important Unit Conversion:

25 microns (μm) = 1 / 1,000 of an inch

Also see metric system