A ray is a portion of a line formed by one endpoint and all the points of the line on the same side of the endpoint. A ray with starting point $A$ and passing through point $B$ is denoted as $\overrightarrow{AB}.$ Unlike a segment, a ray extends infinitely in one direction.

Notice that the rays $\overrightarrow{AB}$ and $\overrightarrow{BA}$ are different although they both lie on the same line. When two rays share the same starting point and form a line, they are said to be opposite rays.