What is the difference between Ohmic and Non-ohmic devices?
All non-ohmic means is that the V and I relationship does not follow Ohm's Law.

Be aware that this is more a description of their behaviour, than a definition of what they are. And conductors that are described as ohmic in one context may be called non-ohmic in another.

A diode is perhaps the classic non-ohmic device, others include filament light bulbs, electrochemical cells, electric arcs. You will note that the short list of conductors here includes a semi-conductor, a metal, a liquid and a gas/plasma, so non-ohmic is hardly a definition of the conductor.

A filament light bulb is interesting, because as its temperature varies from 300K to 3000K, the resistance varies by more than a factor of 10, very non-linear / non-ohmic indeed.

This non-linear resistance trait is shared by most metals, however we don't usually notice it. For instance a copper cable's resistance increases by 0.4% per degree, much the same as tungsten. But we tend to call copper 'ohmic' as we typically to use it to carry currents that don't heat it up much (if your mains extension lead is warm to the touch, then maybe it's close to being overloaded), but tungsten filaments 'non-ohmic' as their normal use is over a much wider range.

It's their behaviour, and whether we define 'follow Ohm's Law' to mean within 1% or 10% or 0.01% accuracy.

But I think everyone without exception would call a diode 'non-ohmic'.