Why is a light bulb not an ohmic resistor?

A light bulb is not an ohmic resistor because its resistance changes with temperature. As the filament heats up when current passes through it, its resistance increases, violating Ohm's law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points. In contrast, ohmic resistors maintain a constant resistance regardless of temperature or current. Thus, the relationship between voltage and current in a light bulb is nonlinear.