Zero initial condition in time-invariant passive linear systems refers to the state where all capacitors have zero initial voltage and all inductors have zero initial current, indicating no stored energy in the circuit. This condition ensures that the system's behavior is solely determined by the inputs applied to it. It simplifies the analysis of the circuit's response, as it eliminates any influence from prior states. Understanding this concept is crucial for accurately modeling and predicting circuit behavior. Overall, zero initial condition is fundamental for analyzing linear systems effectively.