A scan chain is a testing technique used in digital circuits, particularly in integrated circuits like microprocessors. It allows for easier detection of faults by connecting flip-flops in a series. This configuration enables the shifting of test data into the flip-flops and the observation of output data, facilitating the identification of defects in the circuit. In a scan chain, the normal operation of the circuit is modified to include a scan mode. During this mode, the circuit can be tested without affecting its functionality. This method is essential for ensuring the reliability of complex systems and is widely used in the semiconductor industry for design-for-testability purposes.