1. The opposition that a device or material offers to the flow of direct current, equal to the voltage drop across the element divided by the current through the element.
2. The opposition to a flow of electric current through a circuit component, medium, or substance.

It is the magnitude of the actual part of the impedance and is measured in ohms.

An electric furnace in which the heat is developed by the passage of current through a suitable internal resistance that may be the charge itself, a resistor embedded in the charge, or a resistor surrounding the charge.