abbreviation Ω, unit of electrical resistance in the metre-kilogram-second system, named in honour of the 19th-century German physicist Georg Simon Ohm (Ohm, Georg Simon). It is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere (1Ω = 1 V/A); or, the resistance in which one watt of power is dissipated when one ampere flows through it. Ohm's law (q.v.) states that resistance equals the ratio of the potential difference to current, and the ohm, volt, and ampere are the respective fundamental units used universally for expressing quantities.

Impedance (electrical impedance), the apparent resistance to an alternating current, and reactance, the part of impedance resulting from capacitance or inductance, are circuit characteristics that are measured in ohms. The acoustic ohm and the mechanical ohm are analogous units sometimes used in the study of acoustic and mechanical systems, respectively.