a ray
A beam of light leaving a flashlight starts at the bulb and shoots off into the distance without ever turning back. It has a clear starting point but no end. That one-way, half-infinite straight path is what geometry calls a ray.
A ray has a single endpoint, called its origin or initial point, and extends without bound in exactly one direction. We name it ray AB, where A is the endpoint and B is any other point on it that fixes the direction; the arrow in the symbol points away from A through B. Note the order matters: ray AB and ray BA share the same line but start at opposite ends and point opposite ways, so they are different rays. Together they make up the whole line through A and B.
Rays are the building blocks of angles: an angle is formed by two rays sharing a common endpoint, the vertex. Two rays that point in exactly opposite directions from a shared endpoint are called opposite rays, and they form a straight line — and a straight angle of 180 degrees. The idea of betweenness lets us pin a ray down precisely: ray AB is point A together with every point P such that A is not between P and B, capturing 'everything on the B side of A and beyond'.
Ray AB and ray AC sharing the endpoint A form the angle BAC. If instead B and C lie on opposite sides of A in a straight line, ray AB and ray AC are opposite rays, and angle BAC is a straight angle measuring 180 degrees.
Two rays from one vertex make an angle; two opposite rays make a straight line.
Order is built into the name: ray AB starts at A, ray BA starts at B — they are not the same ray even though they lie on the same line.