Einstein did not just pick this term out of a hat. The term relativity comes in when you compare two people's reference frames. But contrary to popular belief in Einstein's time, light is not relative. Effects of Relativity Relativity has special effects that confine matter from doing some things. Therefore, it would take an infinite amount of energy to get something to go the speed of light.
Time dilation is a very hard concept to explain in depth.
You notice that my clock is going slower than yours. But as I look at you through the window of my train at you, I also notice something strange. I notice that your clock is going slower than mine. You see my clock going too slow, and I see your clock going too slow. In a way, yes.