Micron
One thousandth of a millimetre. The unit polyethylene thickness should be quoted in, because it is unambiguous.
Also called: µm, micrometre
A micron figure alone still does not identify a product: 20 µm of HDPE and 20 µm of LDPE behave very differently, and 20 µm of stretch film is a third thing again. Always pair the thickness with the polymer.