The milliradian was first identified in the mid nineteenth Century.
From the en.wikipedia.org
All versions of the angular mil are approximately the same size as a trigonometric milliradian.
From the en.wikipedia.org
The mil is approximately equal to a milliradian.
From the en.wikipedia.org
More examples
A unit of angular distance equal to one thousandth of a radian
The radian is the standard unit of angular measure, used in many areas of mathematics. It describes the plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc. ...