What is Setup and Hold time of a Flip-Flop?

To guarantee correct data propagation in sequential cells, the data input signal must remain stable for a minimum length of time before the active edge of the clock. This period of time is called the setup time. Setup time is measured as the difference between the clock edge and data edge. The hold time is the minimum length of time the input data signal must remain stable after the active edge of the clock. Hold time is measured same as setup.