platoon (n.)

1630s, "a small body of soldiers acting together but separate from the main body of troops," from French peloton "platoon, group of people," literally "little ball" (15c.), hence, "agglomeration," diminutive of Old French pelote "ball" (see pellet). Football sense of "group of players trained to act as a unit on the field" is by 1941.

in baseball, "to alternate (a player) with another in the same position," 1967, from platoon (n.), which had been used in team sports since 1941.