Definition

The Ohm (\Omega) is the unit of resistance and is a measure of a component's opposition to the flow of current (A).

In SI units, an ohm is defined as:

\Omega = kg \cdot m^2 \cdot s^{-3} \cdot A^{-2}

For non-linear devices, the differential definition of resistance is:

r = \frac{d V}{d I}