Why Impedance is the AC generalization of resistance, and like resistance it is defined by Ohm's law as voltage divided by current: Z = V/I. It combines resistance and reactance into one complex quantity measured in ohms, and it also carries a phase angle describing how much the current leads or lags the voltage. Doubling the applied voltage across a fixed impedance doubles the current, keeping the ratio constant.
Watch out The current-to-voltage ratio is the reciprocal, which is admittance (measured in siemens), not impedance. Multiplying voltage by current gives power in watts, not ohms.
Z = E/I, same shape as R = E/I. Ohms come from volts over amps, never amps over volts.