The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Simon Ohm.
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The ohm is defined as the resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.^{[1]}
In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters; one speaks of linear resistors. In other cases resistance varies (e.g., thermistors).
Commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, and megohm.^{[2]}
The power dissipated by a linear resistor may be calculated from its resistance, and voltage or current. The formula is a combination of Ohm's law and Joule's laws:
where P is the power in watts, R the resistance in ohms, V the voltage across the resistor, and I the current through it.
This formula is applicable to devices whose resistance varies with current.
Care should be taken when preparing documents (including HTML documents) which make use of the symbol Ω. Some document editing software will attempt to use the symbol typeface to render the character. Where the font is not supported, a W is displayed instead. As this represents the SI unit of power, not resistance, this can lead to confusion.
Unicode encodes an ohm symbol distinct from Greek omega among Letterlike Symbols.
Gigohm is another example of dropping the extra vowel. Gigohm, Megohm, and Kilohm are commonly used in the literature and machinery within the high voltage test industry. Vitrek Corporation of San Diego, CA is an example of a company which commonly uses this terminology.

An Ohm (referred to by the Greek letter Ω (Omega)) is the SI unit of resistance (R). It can be defined by Ohm's Law and the equation:
where R is Resistance in Ohms V is Volts and A is Amperes.
