Ray

The locus of a point in a direction from a position in a plane is called a ray.

A ray is a one directional straight line. It is actually formed by a point which starts from a position on a plane and continues its traveling infinitely in a direction. It seems, it is half of the straight line.

Formation

$\small R$ is a location in a plane.
A point is started travelling from location $\small R$ and continued travelling infinitely in the same direction by passing through various points including $\small S$.
The locus of the travelling point from a location in a particular direction formed a straight path and it is called a ray.

Representation

A special mathematical representation is used to denote a ray in mathematics and it is developed as per the geometric formation of the ray.

Write the names of two points one after one but the name of the first point should be the point where the straight path is started.
Display a right arrow symbol $(\rightarrow)$ over the names of both points which play a vital role in forming the ray.

In this example, $R$ is a starting point $S$ is any point on the straight path. Write the names of them as $RS$ and display a right arrow symbol over names of both points.

Therefore, $\overrightarrow{RS}$ is the mathematical representation of a ray. Thus, any ray can be denoted mathematically in this manner.