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The ratio of voltage across to the current in a component.
1. Resistance and Ohm's Law
V is always equal to IR, but V is not always proportional to I. If that doesn't make sense then you need to see this video and the next one on IV characteristics.
2. Resistance and Temperature
Why does the resistance of a conductor increase with temperature? This model of the vibrating ions is one way to visualise what is happening inside the lattice.
Just like resistance is an indication of how much current is impeded, conductance tells us how well something conducts electricity.
(This is not needed for all exam boards - check with your specification)
4. Liquid Nitrogen and High Voltages
As you cool a substance its resistance decreases (if you go extremely low you get into the realms of superconductivity where R = 0).