An electronic component with two terminals, made of resistive materials and having a specific structural form, that functions to limit the flow of current in a circuit. A resistor with an unchangeable resistance value is called a fixed resistor. A resistor with a variable resistance value is known as a potentiometer or variable resistor. An ideal resistor is linear, meaning the instantaneous current through the resistor is directly proportional to the instantaneous voltage applied. Some special types of resistors, such as thermistors, varistors, and sensitive elements, have a nonlinear relationship between voltage and current. Resistors are widely used components in electronic circuits, usually available in different series based on power and resistance for circuit designers to choose from. Resistors are primarily used in circuits to regulate and stabilize current and voltage, serving as shunt resistors and voltage dividers, and can also act as matched loads for circuits. Depending on circuit requirements, they can be used for negative or positive feedback in amplification circuits, voltage-to-current conversion, and as protective elements for voltage or current during input overload. They can also form RC circuits for oscillation, filtering, bypassing, differentiation, integration, and time constant elements.