The zero-input response of a circuit refers to the response of the circuit when all external inputs are set to zero, meaning that any independent sources (like voltage or current sources) are turned off or removed. This response is solely due to the initial conditions stored in the energy storage elements, such as capacitors and inductors.
In other words, the zero-input response captures how the circuit behaves over time due to its inherent characteristics, like resistance, capacitance, and inductance, after an initial disturbance has occurred and without any additional external forcing functions. It's an important concept in the analysis of dynamic systems and is often used in conjunction with the zero-state response to fully describe the circuit's behavior.