In geometry, a ray is a line-like figure that starts at some point - called the "endpoint" or "vertex" - and continues indefinitely in one direction. It is described in formal notation using 2 points with an arrow above it pointing in the direction of the ray, for example: .
The order of the endpoints describes the direction of the ray. It cannot be inversed.
As an example, this is the ray in Ascii art:
A ---- B ->