Micron

A measure of length equal to one millionth of a meter.

FAQ
Micron: a measure of length equal to one millionth of a meter.
The contaminants and particles that are attempting to be filtered out are much too small to see with a low powered microscope or a magnifying glass. A general way to quantify the ability of the filters media to remove contaminants of a certain size is known as a fuel filter micron rating.
1 micrometre