Background
Ohm discovered that the voltage (V) across a resistor changes as the current (I) passes through the resistor. The relationship between V and I can be expressed as

where R is the resistance of the resistor. In other words, as the voltage increases, so does the current. The proportionality constant is the value of the resistance.

Ohmic material
A resistor is 'ohmic' if the voltage across it varies with the current as a linear relationship. The slope of a graph of V against I is the value of the resistance,

where the slope remains constant.

Non-ohmic material
For a light bulb, the resistance of the filament will change as it heats up and cools down. At high a.c. frequencies, the filament doesn't have time to cool down, so it remains at a nearly constant temperature and the resistance stays relatively constant. At low frequencies, the filament has time to change temperature. As a result, the resistance of the filament changes dramatically.