The mil is a unit of length in the imperial unit system and uses the symbol mil (or sometimes thou).
A mil in one one-thousandth of an inch (0.001 inches) or 25.4 μm.
The mil is still used in the US to measure thickness of various materials.
The light year is a unit of length in the Astronomical system of units. It uses the symbol ly.
Often confused with a unit of time as it contains the word "year", it is actually defined by the International Astronomical Union (IAU) as the amount of distance that light travels in a vacuum in one Julian year (365.25 days).
Using the speed of light (299,792,458 m/s or 'c') and the Julian year (as apposed to the Gregorian year which is 365.2425 days), we can equivalate 1 light year as 9,460,730,472,580,800 metres or ≈ 9.46 trillion kilometres.