Non-ohmic devices don't follow V = IR (V-I graph is non-linear). Examples: semiconductor diode (allows current in one direction only; forward voltage drop ~0.6 V for Si), Zener diode (constant voltage in reverse breakdown), thermistor (R changes significantly with T — NTC or PTC), LDR (R decreases with light intensity), filament lamp (R increases as it heats up — V-I curve is concave). For these devices, resistance at a point is the slope V/I (static resistance) or dV/dI $\frac{dynamic}{differential resistance}$. JEE problems involving these devices typically require graphical analysis or piecewise linear models.