Why are the trigonometric functions sometimes called circular functions?

Trigonometric functions are called circular functions because function values can be expressed in terms of coordinates of a point on a unit circle.
These trignometric functions are called circular functions because radian measures of angles are determined by the lengths of arcs of circles.

The sine and cosine function are defined as the coordinates of a point P on a circle of radius 1.
${\displaystyle x=\cos \theta \text{and}y=\sin \theta }$.
Hence, trigonometric functions are called circular functions because function values can be expressed in terms of coordinates of a point on a unit circle.