A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.

विद्युतप्रवाह रोधणाऱ्या, अर्थात विद्युतप्रवाहास अडथळा आणणाऱ्या, घटकाला रोधक म्हणतात. रोधकातून विद्युतप्रवाह वाहवण्यासाठी त्याच्या टोकांदरम्यान विद्युतदाब लावावा लागतो. रोधकाच्या विद्युतप्रवाह रोधण्याची क्षमतेला रोध असे म्हणतात. विद्युतदाब, विद्युतप्रवाह व रोध यांचा संबंध ओहमाच्या नियमानुसार खालील सूत्रात मांडला जातो :