A whip is an official of a political party whose task is to ensure party discipline (that members of the party vote according to the party platform rather than their constituents, individual conscience or donors) in a legislature.

Whips are the party's vote organisers and go-betweens. They work to ensure that their fellow political party legislators attend voting sessions and vote according to their party's official policy. Members who vote against party policy may "lose the whip,” being expelled from the party.

The term is said to be taken from the "whipper-in" during a hunt, who tries to prevent hounds from wandering away from a hunting pack.

Additionally, the term "whip" may mean the voting instructions issued to legislators, or the status of a certain legislator in their party's parliamentary grouping.

The expression whip in its parliamentary context, derived from its origins in hunting terminology. The Oxford English Dictionary defines the term whipper-in as, "a huntsman's assistant who keeps the hounds from straying by driving them back with the whip into the main body of the pack". According to that dictionary, the first recorded use of the term whipper-in in the parliamentary sense occurs in 1772. However, P.D.G. Thomas in House of Commons in the Eighteenth Century cites two examples of the use of the term that pre-date 1772.

It was within the context of such summonses to members out of town that the first known Parliamentary instance of the use of the term "whip" occurred. In the debate of 8 May 1769 on a petition from some Middlesex freeholders against the seating of Henry Luttrell instead of John Wilkes, Edmund Burke mentioned that the ministry had sent for their friends to the north and to Paris, "whipping them in, than which, he said, there could not be a better phrase". Although Burke's particular emphasis on the expression implied its comparative novelty, the hunting term had been used in this political context for at least a generation: on 18 November 1742 Heneage Finch remarked in a letter to Lord Malton that "the Whigs for once in their lives have whipped in better than the Tories".

In the Parliament of Australia, as well as in the parliaments of the six states and two self-governing territories, major political parties have whips to ensure party discipline and carry out a variety of other functions on behalf of the party leadership. The most important function of the whip's office is to ensure that all members and senators are present to take part in votes in the chamber (maintaining quorum and preventing censure motions). Unlike in the United Kingdom, Australian whips do not hold official office, but they are recognised for parliamentary purposes. In practice, Australian whips play a lesser role than their counterparts in the United Kingdom, as party discipline in Australia tends to be tighter.

Their roles in the chamber include taking divisions, and maintaining a "pairs book" which controls the ability of members and senators to leave the parliament building during sittings, as well as the entitlement to be absent during divisions.