the name of two kings of England and two of Great Britain and Ireland. William I (circa 1027–87), reigned 1066–87; the first Norman king of England; known as William the Conqueror. He invaded England and defeated Harold II at the Battle of Hastings (1066). He introduced Norman institutions and customs (including feudalism) and instigated the Domesday Book. William II (circa 1060–1100), son of William I; reigned 1087–1100; known as William Rufus. He crushed rebellions in 1088 and 1095 and also campaigned against his brother Robert, Duke of Normandy (1089–96), ultimately acquiring the duchy. William III (1650–1702), grandson of Charles I, husband of Mary II; reigned 1689–1702; known as William of Orange. In 1688, he deposed James II at the invitation of disaffected politicians and was crowned along with his wife Mary. William IV (1765–1837), son of George III; reigned 1830–37; known as the Sailor King. Having served in the Royal Navy, he came to the throne after the death of his brother George IV.