Phasor addition is the mathematical technique used to combine phasors, which are complex numbers representing sinusoidal signals in the frequency domain. This method leverages the principles of vector addition, allowing for the straightforward calculation of resultant voltages or currents when dealing with multiple sinusoidal sources. By converting sinusoidal waveforms into their phasor equivalents, phasor addition simplifies the analysis of circuits involving alternating current (AC) signals.