A phasor is a complex number that represents the amplitude and phase angle of a sinusoidal function, transforming it from the time domain to the complex number domain. Phasors were introduced by electrical engineer Charles Proteus Steinmetz in the early 20th century and are defined by the IEEE as a complex number that can represent the amplitude and phase angle of a sinusoidal function. Phasors are useful for analyzing circuits containing inductors and capacitors where the current and voltage may be out of phase.

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A phasor is a complex number that
represents the amplitude and phase angle of
a sinusoidal function.
It transforms a sinusoidal function from time
domain to the complex number domain
The IEEE Dictionary (IEEE 100) defines a
phasor as ‘‘a complex number.’’
Phasors are credited to electrical engineer
Charles Proteus Steinmetz (1865–1923)

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Recall

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Capacitor - current leads voltage
Voltage leads current - Inductor