In computer science, the propogation delay is the amount of time starting from when the input to a logic gate becomes stable and valid to the time that the output of that logic gate is stable and valid.
In physics, particularly in the Electromagnetism field, the propagation delay is the amount of time it takes for a signal to travel to its destination. For example, in the case of an electric signal, it's the time it takes to travel through a wire. See Velocity_of_propagation.