Frame of reference
This article needs to be wikified. (November 2011)
A frame of reference is how one knows if an object is moving. For example, when you see a ball roll down a street, you can tell the ball is moving because the frame of reference is the streets, whatever may be on the side of the roads or the Earth. All of these are a frame of reference.
The most common frame of reference is Earth itself, even though it moves.