A measurement of the opposition of a body or substance to current passing through it. Resistance causes a change of electrical energy into heat or another form of energy. All electrical circuits and wires have some resistance to the passage of electricity. The unit of the resistance is the ohm (Ω), named after George Ohm, who formulated Ohm’s Law. Impedance, a special case of resistance for AC signals, also uses the ohm as its unit of measurement.