Impedance is defined as the measure of the degree to which an electric circuit resists electric-current flow when a voltage is impressed across its terminals. Impedance, which is expressed in Ohms, is the ratio of the voltage impressed across a pair of terminals to the current between those terminals. In direct-current (DC) circuits, impedance corresponds to only resistance. In alternating current (AC) circuits, impedance is a function of resistance, inductance, and capacitance.
So if you are soley dealing with DC circuits, then resistance will be equal to impedance. When dealing with AC circuits, then you will have real and imaginary components of impedance to deal with. Our devices actually measure voltage and to measure resistance the device is basically measurin
g voltage and then using Ohm's Law (voltage = current * resistance) to calculate resistance.
