What's the correct term to describe a conductor which doesn't always obey Ohm's law?
I've heard such things as thermistors called "non-linear resistors":

Thermistor

The idea is that a linear resistor has a linear V-I curve, but a non-linear resistor does not. In the case of a thermistor, it's due to heating, but in the case of, say, a varistor, it could be due to voltage:

Varistor