"Mil" is short for "thousandth of an inch." Not million, not millimeter, though everyone guesses one of those first. The unit dates back to 1857, when a British engineer named Joseph Whitworth got sick of machinists arguing over sixteenths and thirty-seconds and told them to start thinking in decimals. It stuck, and now it shows up everywhere from paint coatings to wire gauge to sticker vinyl.
To try to make it relatable, here's a quick chart of the various thicknesses of vinyl we use.
