Lorentz factor
Classical or Galilean relativity is the idea that if you throw a ball at 50 mph while running at 5 mph, the ball travels 55 mph. Of course, the ball still moves away from you at 50 mph, so if one were to ask you, you saw the ball traveling 50 mph. Meanwhile, your friend Rory saw that you happened to be running at 5 mph. He would say that the ball was traveling 55 mph. Both of you are right, you just happened to be moving with the ball.