A resistor is a passive element or component because it does not produce any energy. A resistor can only consume the energy in a circuit, and dissipate the energy. Whenever a resistor exists in an electrical circuit, it opposes the current flow and converts electrical energy into heat. Resistors cannot amplify signals, nor can they generate power, as opposed to active components like operational amplifiers or transistors. The role of a resistor is simply to regulate current in a circuit to make it easier for circuit designers and engineers to maintain voltage or keep sensitive components from burning up.
Resistors are passive because they do not need an external power supply to function. They do not turn on or activate, when in a circuit they do their job by merely existing in the circuit, and have a value of resistance, measured in ohms. Resistors operate in a mostly linear, predictable, and uniform manner no matter what signal they are taking or transforming. Resistors are also reliable, consistent, and easy to work with in an nearly any electronic application.
Additionally, resistors do not exhibit any way to dynamically or time-varyingly control energy flow. Active components can control signals and also react to them in complex ways while usually requiring some sort of power supply. Resistors have neither such properties; they also do not store energy like capacitors and inductors do. Resistors simply provide resistance to current to contribute to steady and controlled performance of the circuit.
In summary, the defining feature of a passive component is the inability to inject energy into a circuit, and the resistor is the best example of this. Resistors neither supply power nor adjust their operation based on the energy they receive. The fundamental predictability of resistors makes them one of the foundational building blocks in both analog and digital circuits which make them incredibly useful for current limiting, signal level setting, and power dissipation.
