Resistance is the opposition which a substance offers to flow of electric current. It is represented by letter R. Standard unit of resistance is the ohm.
When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then resistance of that component is one ohm.
In general, when applied voltage is held constant, current in a direct-current (DC) electrical circuit is inversely proportional to the resistance. If resistance is doubled, current is cut in half; if resistance is halved, current is doubles. This rule also holds true for low-frequency alternating-current (AC) systems like household utility circuits. In some Ac circuits, at high frequencies, situation is more complex because some components I these systems can store and release energy and dissipating or converting it.
Electrical resistance per unit length, area or volume of a substance is called ‘resistivity’. Resistivity figures are specified for copper and aluminum wire, in ohms per kilometer.
Opposition to AC but not to DC, is a property known as ‘reactance’. In an AC circuit, resistance and reactance combine vectorially to yield impedance.
Resistance contrasts with conductance, which is a measure of the ease with which electrical current flows through a substance.