(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.]
(by extension) A new set of anything.
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.]