Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics. In general it is the length of time taken for the quantity of interest to reach its destination.
In computer networks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver over a medium. It can be computed as the ratio between the link length and the propagation speed over the specific medium.
Propagation delay = d/s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wires, the speed s is typically about 67% av of speed of light. This delay is the major obstacle in the development of high-speed computers and called interconnect bottleneck in IC systems.
In physics, particularly in the electromagnetism field, the propagation delay is the length of time it takes for a signal to travel to its destination. For example, in the case of an electric signal, it is the time taken for the signal to travel through a wire. See also, velocity of propagation.