Whiggism or Whiggery is a political philosophy that grew out of the Parliamentarian faction in the Wars of the Three Kingdoms (1639–1653) and was concretely formulated by Lord Shaftesbury during the Stuart Restoration. The Whigs advocated the supremacy of Parliament (as opposed to that of the king), government centralisation, and coercive Anglicisation through the educational system. They also staunchly opposed granting freedom of religion, civil rights, or voting rights to anyone who worshipped outside of the Established Churches of the realm. Eventually, the Whigs grudgingly conceded strictly limited religious toleration for Protestant dissenters, while continuing the religious persecution and disenfranchisement of Roman Catholics and Scottish Episcopalians. They were particularly determined to prevent the ascension of a Catholic heir presumptive to the British throne, especially of James II or his legitimate male descendants and instead granted the throne to the Protestant House of Hanover in 1714. Whig ideology is associated with early conservative liberalism.

Beginning with the Titus Oates plot and Exclusion Crisis of 1679–1681, and the Glorious Revolution of 1688–1689, Whiggism dominated English and British politics until about 1760, after which the Whigs splintered into different political factions. In the same year, King George III was crowned and allowed the Tories back into the Government. Even so, some modern historians now call the period between 1714 and 1783 the, "age of the Whig oligarchy".

Even after 1760, the Whigs still included about half of the newest noble families in England, Ireland, Wales, and Scotland, as well as most merchants, dissenters, and the middle classes. The opposing Tory position was held by the other great families, the non-juring and high church factions within the Church of England, many Catholics and Protestant Dissenters, most of the landed gentry and the traditional officer class of the British armed forces. Whigs especially opposed regime change efforts by adherents of Jacobitism, a movement of legitimist monarchists which promised freedom of religion and civil rights to all outside the Established Churches, devolution in the United Kingdom, linguistic rights for minority languages, and many other political reforms, and which shared a substantial overlap with and heavily influenced both early Toryism and what is now termed traditionalist conservatism. While in power, Whig politicians frequently denounced all their political opponents and critics as "Jacobites" or "dupes of Jacobites".

The terms "Old Whigs" and Patriot Whigs were also used in Great Britain for those Whigs who formed a de facto coalition with the Tories and jointly opposed Robert Walpole as part of the Country Party. Whiggism originally referred to the Whigs of the British Isles, but the name of "Old Whigs" was largely adopted by the American Patriots in the Thirteen Colonies of British North America. Before and during the American Revolution, American Whiggism, in a deeply ironic reversal, weaponised Whig political philosophy about the social contract enforced by the right of revolution against both the Whig-dominated government in Westminster and the Hanoverian monarchs. In the process, American Whiggism ultimately transitioned from monarchism into republicanism and Federalism, while also co-opting many traditionally Jacobite, Counter-Enlightenment, and early Tory positions. A similar but far more discreet co-opting was also taking place in the British Isles among many self-described Whigs, including Edmund Burke, Henry Grattan, William Wilberforce, Daniel O'Connell, and William Pitt the Younger. Even though they were often influenced in this regard by the writings of early Tories and other intellectual critics of the Whig party like Jonathan Swift, Lord Bolingbroke, and David Hume, these reformist Whigs, similarly to American Patriots, refused to use the word "Tory" as anything other than a term of abuse against those with more traditionalist Whig ideology, which ultimately changed the word's meaning completely. Whig history, which was largely developed by Thomas Babington Macaulay to justify the party's political ideology and past practices, remained the official history of the British Empire until serious challenges were raised to its claims by John Lingard, William Cobbett, Hilaire Belloc, G. K. Chesterton, Roger Scruton, Saunders Lewis, and John Lorne Campbell.

Quickly following the adoption of "Whig" as the name of a political faction, the word "Whiggism" arose from the appendage of the suffix "-ism", creating a term for the Whigs' political ideology. It was already in use by the 1680s. In 1682, Edmund Hickeringill published his History of Whiggism. In 1702, writing satirically in the guise of a Tory, Daniel Defoe asserted: "We can never enjoy a settled uninterrupted Union and Tranquility in this Nation, till the Spirit of Whiggisme, Faction, and Schism is melted down like the Old-Money". The name probably originates from a shortening of Whiggamore referring to the Whiggamore Raid.

The word "Whiggery", deriving from "Whig" and the suffix "-ery", has a similar meaning and has been used since the late 17th century.

The true origins of what became known as Whiggism lie in the Wars of the Three Kingdoms and the power struggle between the Parliament of England and King Charles I, which eventually turned into the English Civil Wars, but only after the example of the successful use of violent opposition to the king set by the Bishops' Wars, which were fought between the same king in his capacity as king of Scotland on the one side and the Parliament of Scotland and the Church of Scotland on the other. However, the immediate origins of the Whigs and Whiggism were in the Exclusion Bill crisis of 1678 to 1681, in which a country party battled a court party in an unsuccessful attempt to exclude James, Duke of York, from succeeding his brother Charles II as king of England, Scotland and Ireland. This crisis was prompted by Charles's lack of a legitimate heir, by the discovery in 1673 that James was a Roman Catholic, and by the so-called Popish Plot of 1678.

While a major principle of Whiggism was opposition to popery, that was always much more than a mere religious preference in favour of Protestantism, although most Whigs did have such a preference. Sir Henry Capel outlined the principal motivation of the cry of "no popery" when he said in the House of Commons on 27 April 1679: