The ohm is the SI unit of electrical resistance.
One ohm is the resistance between two points of a conductor when a potential difference of one volt produces a current of one ampere. In symbols, resistance is measured in ohms, written with the Greek capital letter omega, Ω. The relationship is expressed by Ohm's law: R = V/I.
The unit honors Georg Ohm, the German physicist who published important work on the relationship between voltage, current, and resistance in 1827. Resistance describes how strongly a material or component opposes electric current, but it is not the same as voltage or power.
A resistor's value can change with temperature, length, cross-sectional area, and material. Metals generally become more resistive as they heat, while some semiconductor devices behave differently. The watt measures power, the volt measures potential difference, and the farad measures capacitance.