Ohmic and Non-ohmic resistances(devices)

Resistances that obey the Ohm's law are called ohmic resistances. A metallic conductor at a constant temperature is called Ohmic resistance. For such ohmic resistances, the V-I characterstic curve will be a straight line passing through the origin.
In ohmic resistance, current is reversed in the direction when the potential difference is reversed, but the magnitude of the current remain the same.

There are many resistances that do not obey the Ohm's law. These are called Non-ohmic resistances. For these resistances, the V-I characterstic curve will not be linear. The function of modern electronic devices that do not obey the Ohm's law.