Visualized motion vectors. The foreground character's rotating downward head movement is visible, as well as the background character's slower upward head movement.
In video compression, a motion vector is the key element in the motion estimation process. It is used to represent a macroblock in a picture based on the position of this macroblock (or a similar one) in another picture, called the reference picture.
The H.264/MPEG-4 AVC standard defines motion vector as:
motion vector: A two-dimensional vector used for inter prediction that provides an offset from the coordinates in the decoded picture to the coordinates in a reference picture.