Short answers, pulled from the story.
Oliver Heaviside coined the term impedance in July 1886 within his operational calculus framework. He recognized that the resistance operator was actually a complex number representing an AC equivalent to Ohm's law.
The unit for this measurement is the ohm, which remains consistent with standard resistance units despite the added complexity. Engineers use this ratio to analyze how circuits behave when driven by sinusoidal voltages at specific frequencies.
An inductor's impedance increases as frequency rises because its back electromotive force opposes changing currents. In an inductor, current lags voltage by ninety degrees while in a capacitor, current leads voltage by the same amount.
Charles Proteus Steinmetz generalized Kennelly's work to all AC circuits later that same year after Arthur Kennelly published an influential paper on impedance in 1893. Steinmetz represented impedances, voltages, and currents by complex numbers to express AC equivalents of DC laws like Kirchhoff's laws.
Electrical engineering uses the letter j for the imaginary unit instead of i to avoid confusion with electric current I. Cartesian form defines impedance as R plus jX where R is resistance and X is reactance.