Each such part is called a ray and the point "A" is called its "initial point". It is also known as half-line, a one-dimensional half-space. The point A is considered to be a member of the ray. Intuitively, a ray consists of those points on a line passing through "A" and proceeding indefinitely, starting at "A", in one direction only along the line. However, in order to use this concept of a ray in proofs a more precise definition is required.