resistivity

Resistivity is a material's built-in stinginess about letting charge flow, a property of the stuff itself rather than of any particular wire cut from it. Copper is generous and lets current pass easily; rubber is miserly and almost blocks it. Resistivity answers the question a materials scientist cares about: forgetting shape and size, how good or bad a conductor is this substance, gram for gram?

Precisely, resistivity rho is defined so that a sample's resistance is R = rho L / A, where L is length and A is cross-sectional area. Turned around, rho = R A / L, and its unit is the ohm-metre. Because it strips out the length and thickness, rho lets you compare copper with iron or silicon fairly, independent of how the wire was shaped. Its reciprocal is the conductivity sigma = 1 / rho, which measures how readily a material conducts. Metals have tiny resistivities (copper is about 1.7 x 10^-8 ohm-metres), insulators have colossal ones, and semiconductors sit in between.

Resistivity is not a fixed constant even for one material: for most metals it rises as temperature goes up, because hotter atoms jiggle more and get in the electrons' way, which is exactly why a bulb filament's resistance climbs when lit. Semiconductors do the opposite, conducting better when warmer, and some materials drop to zero resistivity entirely below a critical temperature, becoming superconductors. So when a resistivity is quoted, it is usually the value at a stated temperature, often 20 degrees Celsius.