The ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and current, expressed in Ohm's Law.

By definition from Ohm's Law, a device has a resistance of one ohm if a voltage of one volt will cause a current of one ampere to flow (R = V/I). Alternatively and equivalently, a device that dissipates one watt of power with one ampere of current flowing through it has a resistance of one ohm (R = P/I²).

One thousand ohms is called a kilohm or kiloohm, and abbreviated kΩ. One million ohms is called a megohm or megaohm, and abbreviated MΩ. Other orders of magnitude follow standard SI conventions, and the last letter of the prefix is rarely omitted.

A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance. Note that 'siemens' is both singular and plural. The reciprocal of the ohm is also called the mho, from ohm written backwards.

The complex quantity impedance is a generalisation of resistance. Its real part is resistance and its imaginary part is reactance. For consistency, impedance, resistance and reactance all have units of ohms.

The ohm is since 1990 maintained internationally using the quantum Hall effect, where a conventional value is used for the 'von-Klitzing constant', fixed by the 18th CGPM as R_{K-90} = 25812.807 Ω.

SI electricity units

See also

• Ohm's Law
• Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuremberg

For the Hindu mantra syllable, see aum.