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.

\(\displaystyle{x}={\cos{\theta}}\ \text{and}\ {y}={\sin{\theta}}\).

Hence, trigonometric functions are called circular functions because function values can be expressed in terms of coordinates of a point on a unit circle.