Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens.
Resistance is defined as the ratio of the potential difference (i.e. voltage) across the object (such as a resistor) to the current passing through it:
R = V / I
where
For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current flowing or the amount of applied voltage. This means that voltage is proportional to current and the proportionality constant is the electrical resistance. This case is described by Ohm's law and such materials are described as ohmic. V can either be measured directly across the object or calculated from a subtraction of voltages relative to a reference point. The former method is simpler for a single object and is likely to be more accurate. There may also be problems with the latter method if the voltage supply is AC and the two measurements from the reference point are not in phase with each other.
Source: CDX Global & Wikipedia - en.wikipedia.org
