a) A vector is a directed edge. That is, given points A and B, the line that connects A and B becomes a vector if we specify its direction (i.e., which point is the start point). The vector that goes from A to B is not the same vector as the one that goes from B to A. Vectors exist in 3D; they connect points in 3D space. b) An entity that possesses the attributes of a norm and a direction. It can be defined in 3D space by two points, one representing the origin and the other, the extremity. c) A motion compensation parameter that tells a decoder how to shift part of a previous picture to more closely approximate the current picture.