In electronics terminology, Set-Up Time is a parameter associated with the data input of a sequential device, such as a flip flop.

Sequential circuits rely on a clock signal in order to change state. In the case of a flip flop, for example, the data presented to the data input of the device will not be 'captured' until a clock transition occurs. To avoid metastability, the data presented to the input of the device must be stable when it is being captured.

Set-Up Time is defined as the length of time that data must be stable before the active clock edge in order for that data to be captured cleanly.