Impedance, represented by the symbol Z, is a measure of the opposition to electrical flow. It is measured in ohms.
For DC systems, impedance and resistance are the same, defined as the voltage across an element divided by the current (R = V/I).
In AC systems, the "reactance" enters the equation due to the frequency-dependent contributions of capacitance and inductance. Impedance in an AC system is still measured in ohms and represented by the equation Z = V/I, but V and I are frequency-dependent.