One thousandth of an inch, equal to 0.0254 mm.
The thou is the working unit of inch-based machining — a "half-thou tolerance" is ±0.0005 in, or ±12.7 µm. The equivalent metric colloquialism is the micron (µm), and the two are related by 25.4.
"Mil" means the same thing in North American usage, which causes confusion with the millimetre elsewhere. In a drawing, always write the unit rather than relying on the abbreviation.
Reading "mil" as millimetre in a drawing written to North American convention. A 5 mil tolerance is 0.127 mm, not 5 mm.
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