Light Year: A light year is the distance that light travels in one year through a vacuum, approximately equal to 9.461 trillion kilometers (about 5.878 trillion miles). It's used to measure vast cosmic distances.
Millimeter (mm): The millimeter is equal to one one-thousandth of a meter (0.001 meters). It is frequently used for very precise measurements, such as the thickness of paper or the diameter of small objects.