Time to Transmit a Bit
Propagation Delay: There is always a short but finite time delay for a signal (electrical, optical, or radio) to propagate (or travel) from one end of the transmission medium to the other. This is known as the propagation delay. Although this may seem insignificant, in some situations this delay is important! It can be thought of as the time it takes for a single bit to travel from one point to another, where the two points are the two "ends" of the medium. (See Homeworks for how to calculate the propagation delay!)
Propagation Delay (Alternate definition): Time requiired for data to travel over a network, from its source to its ultimate destination.
Propagation Delay (One more definition): Time taken by a signal to propagate from transmitter to receiver across a communications medium.
Bit Error Rate