Micron (“micro meter”)

A unit of linear measurement equal to one millionth of a meter, one thousandth of a millimeter, or 0.000039 inch. A micron is also called a micrometer.

Aviation Maintenance Technician Handbook — Airframe, p. G-22

Also defined in 1 more source

A measurement used to identify the size of particles trapped by filters. One micron is a micro meter, or one millionth of a meter. It is 0.000039 inch.

Aviation Maintenance Technician Handbook — Powerplant, p. G-18