A congressional whip is an assistant party leader whose primary function is to count heads, gather party members for votes and quorum calls, and stand in for the party leader when during an absence. A whip is elected for each party in the Senate and the House.
The term "whip" was shortened from "whipper-in," which originally referred to the member of a fox-hunting team tasked with keeping the dogs from straying during a chase. The Democratic party elected the first party whip in 1913, and the Republicans elected their first two years later. Since 1969, the Republican whip has been called the Assistant Leader.