Ohm’s Law states that the ratio of the potential difference across a conductor to the current flowing through it, is a constant, provided that its physical conditions, such as temperature, remain constant.

Conductors that give a constant ratio of V to I (i.e. a constant value of resistance) are ohmic conductors(obey Ohm’s law).

The resistance of a device is the ratio of the potential difference across it to the current flowing through it. (NOT dV/dI)

$R = \frac{V}{I}$

A resistor has a resistance of one ohm if there is a current of one ampere through it when the potential difference across it is one volt.