Both the Senate and House have majority and minority whips. A whip is a person, whose primary purpose is to ensure party discipline in a legislature. Whips typically offer inducements and threaten punishments for party members to ensure that they vote according to the official party policy.
A whip's role is also to ensure that the elected representatives of their party are in attendance when important votes are taken. The usage comes from the hunting term 'whipping in', such as preventing hounds from wandering away from the pack.