Definition
A unit of length equal to one-thousandth of an inch (0.001 in), used in aviation maintenance to specify small thicknesses, clearances, and tolerances such as sheet metal gauges, paint film thickness, and wire diameters.
Plain English
One mil is one thousandth of an inch — a very small measurement used when something needs to be measured more finely than a regular ruler allows.
Context Anchor
Seen in aircraft maintenance when measuring small thicknesses, clearances, protective coatings, or sheet material.
Derivation
From the Latin 'mille,' meaning 'thousand.' A mil is one one-thousandth of an inch, so the name points directly to the fraction it represents.
Why Pilots Care
Mils show up in inspection limits and repair specifications. Misreading a mil as a millimeter (which is about 39 times larger) can lead to accepting a part that is well outside tolerance or rejecting one that is fine.
Analogy
If one inch were sliced into 1,000 equal pieces, one piece would be one mil.
Intuition Check
Do not read mil as millimeter or as short for military here. In this maintenance context, one mil means 0.001 inch.
Example Sentence 1
The technician measured the paint coating at 4 mils, within the manufacturer's specified range.
Example Sentence 2
The work order required the corrosion-resistant film to be applied at a minimum thickness of 4 mils.