- unit of length equal to 1 millionth of a meter
- used to describe the pore size of filter media; (sand filters have openings of 25 to 30 microns; cartridge filters have openings of 8 to 10 microns; and D.E. (diatomaceous earth) filters have openings of 1 to 5 microns); a granule of table salt is between 90 to 110 microns
