Etymology 1 
From Middle French relai (“reserve pack of hounds”), from relaier (“to exchange tired animals for fresh”); literally, "to leave behind", from Old French relaier (“to leave behind”), from re- + laier (“to leave”), of uncertain origin.
relay (plural relays)
- (hunting, now rare) A new set of hounds. [from 15th c.]
- (now chiefly historical) A new set of horses kept along a specific route so that they can replace animals that are tired. [from 17th c.]
- A series of vehicles travelling in sequence. [from 18th c.]
- (athletics) A track and field discipline where runners take turns in carrying a baton from start to finish. Most common events are 4x100 meter and 4x400 meter competitions. [from 19th c.]
- (electronics) An electrical actuator that allows a relatively small electrical voltage or current to control a larger voltage or current. [from 19th c.]
Derived terms 
- (obsolete, intransitive, hunting) To release a new set of hounds. [15th-17th c.]
- (transitive, now rare) To place (people or horses) in relays, such that one can take over form another. [from 18th c.]
- (intransitive, now rare) To take on a new relay of horses; to change horses. [from 19th c.]
- (transitive) To pass on or transfer (information). [from 19th c.]
- (to relay a message): convey
Etymology 2 
- Alternative spelling of re-lay.