The analysis of ac circuits uses a rotating vector called a

Phasor

Explanation:

A phasor is usually referred to as a rotating vector which has scaled lines. A vector quantity is one which has both magnitude and direction.

Since a phasor is also a vector quantity it has the magnitude and direction in the form of quantity and displacement. The quantity and displacement are usually measured in a counterclockwise direction.

Phasors is used in the engineering world to establish relationships between AC quantities and the parameters such as current and voltage etc.