Negative resistance is the phenomenon in which the current through a device decreases as the voltage increases. This is in contrast to conventional logic in which current increases as the voltage increases.
While negative resistance exists, there is no such thing as a negative resistor. Negative Resistance exists along some are of the V-I curve in certain electronic components such as the Gunn Diode used in microwave electronics.
In certain regions of the V-I curve, the current drops as the voltage rises.