Frame of Reference

The perspective from which an event is viewed.

For example a man is standing on the side of the road, and he sees a woman drive past him in a car. From the man's frame of reference, he is stationary and the woman is speeding past him at 40 miles per hour. However, from the woman's frame of reference, the man appears to be speeding past her in the opposite direction but at the same speed. (This scenario is easier to reason out if there is no "universal stationary", ie. the earth).

Measurements of an event (ie. speed or direction) are taken from an Inertial Frame of Reference, which is an imaginary static point, where events are viewed from. If the speed of a spaceship is to be measured from earth, then the earth is the inertial frame. The earth isn't really stationary, but for the purposes of measuring, it is said to be.