Relating voltage and current
In a simple direct-current measurement, resistance can be expressed as voltage across a component divided by current through it, in ohms. The ratio stays approximately constant for ohmic behaviour under specified conditions. Experimental linearity applies to suitable components, not to every possible electrical device.
References: [2]
Material and geometry
For a uniform wire, resistance depends on length, cross-sectional area and resistivity. Longer comparable wires generally resist more; wider ones less. Resistivity describes material transport under given conditions, whereas resistance belongs to the actual component, enabling samples of different shapes to be compared meaningfully.
References: [1]
Temperature and transport
Interactions with lattice vibrations, impurities and other features transfer electrical energy into internal energy. Temperature affects this transport. Semiconductor carrier populations also change with conditions, so their temperature dependence need not match an ordinary metal’s. Measurements consequently require both material identity and operating temperature.
Components and special states
Resistors intentionally limit current, divide voltage and support other functions, with heating sometimes desired. Low-resistance conductors reduce transmission losses, while superconducting states require their own conditions. Whether resistance is useful or costly depends on the task rather than representing a universal manufacturing defect.