An Etymological Dictionary of Astronomy and Astrophysics

General: The act of passing something along from one person, group, or device to another.
Electronics: An electronic or electromechanical switching device, typically operated by a low voltage, that controls a higher-voltage or higher-power circuit and turns it on or off.
A device consisting of a receiver and a transmitter, used to receive and retransmit signals.

Etymology (EN): M.E. relaien “to unleash fresh hounds in a hunt,” from
M.Fr. relai “reserve pack of hounds or other animals,”
from O.Fr. relaier “to exchange tired animals for fresh,” literally “to leave behind,” from → re- “back” + laier “to leave.”

Etymology (PE): Ask “relay horse kept in stations for the use of messengers,” maybe from asb→ horse.