A phasor is a complex number used to represent sinusoidal functions, particularly in electrical engineering and physics. It simplifies the analysis of alternating current (AC) circuits by converting time-dependent waveforms into a steady-state form. This allows engineers to easily calculate voltage, current, and impedance without dealing with the complexities of time-varying signals. Phasors are typically represented in polar or rectangular coordinates, where the magnitude indicates the amplitude of the waveform and the angle represents its phase shift. By using phasors, one can apply techniques from complex number mathematics to solve problems involving sinusoidal signals efficiently, making them a valuable tool in the study of electromagnetism and signal processing.