Negative resistanceIn electrical circuits static resistance is the ratio of the voltage across a circuit element to the current through it. However, the ratio of the voltage to the current may vary with either voltage or current. The ratio of the voltage differential to the current differential is known as dynamic resistance.
Circuit elements composed of certain materials exhibit the property that, over certain voltage ranges current is a decreasing function of the voltage. This range of voltages is known as a negative resistance region.
It may be more correct to say that a circuit element has a negative resistance region than to say that it exhibits negative resistance because even in this region the static resistance of the circuit element is positive.