The resistance of a length L of conductor with cross sectional area A is defined as

where is the resistivity and is the electrical conductivity. The MKS unit of resistance is the Ohm, denoted ,and s cm_-1 (the cgs unit).
Resistance is related to the voltage V and current I via Ohm's law,

Resistance is temperature-dependent, and to first order given by

where R is a reference resistance at a fixed temperature,is the temperature offset, and is the thermal resistance coefficient.

Resistors in parallel have an effective resistance given by

while resistors in series have an effective resistanc of

Resistance can be generalized to sinusoidal current flow, wave propagation, etc. using the concept of impedance.