The ohm is the unit used as measure of the resistance to flow of electricity in a material. The scientific symbol for electrical resistance, measured in ohms, is the Greek Omega letter (Ω). However, electricians and practical wiring books often use an "R" to represent resistance. This can create some confusion since either may be used.
One ohm 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). It can also be described as the resistance in which one watt of power is dissipated when one ampere flows through it.