Margaret Hilda Thatcher, Baroness Thatcher (née Roberts; 13 October 1925 – 8 April 2013), was Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party from 1975 to 1990. The longest-serving British prime minister of the 20th century, she was the first woman to hold that office. As prime minister, she implemented policies that became known as Thatcherism. A Soviet journalist dubbed her the "Iron Lady", a nickname that became associated with her uncompromising politics and leadership style.

Thatcher studied chemistry at Somerville College, Oxford, and worked briefly as a research chemist, before becoming a barrister. She was elected Member of Parliament for Finchley in 1959. Edward Heath appointed her Secretary of State for Education and Science in his 1970–1974 government. In 1975, she defeated Heath in the Conservative Party leadership election to become Leader of the Opposition, the first woman to lead a major political party in the United Kingdom.

On becoming prime minister after winning the 1979 general election, Thatcher introduced a series of economic policies intended to reverse high inflation and Britain's struggles in the wake of the Winter of Discontent and an oncoming recession. Her political philosophy and economic policies emphasised deregulation (particularly of the financial sector), the privatisation of state-owned companies, and reducing the power and influence of trade unions. Her popularity in her first years in office waned amid recession and rising unemployment, until victory in the 1982 Falklands War and the recovering economy brought a resurgence of support, resulting in her landslide re-election in 1983. She survived an assassination attempt by the Provisional IRA in the 1984 Brighton hotel bombing and achieved a political victory against the National Union of Mineworkers in the 1984–85 miners' strike.

Thatcher was re-elected for a third term with another landslide in 1987, but her subsequent support for the Community Charge ("poll tax") was widely unpopular, and her increasingly Eurosceptic views on the European Community were not shared by others in her cabinet. She resigned as prime minister and party leader in 1990, after a challenge was launched to her leadership. After retiring from the Commons in 1992, she was given a life peerage as Baroness Thatcher (of Kesteven in the County of Lincolnshire) which entitled her to sit in the House of Lords. In 2013, she died of a stroke at the Ritz Hotel, London, at the age of 87.

A polarising figure in British politics, Thatcher is nonetheless viewed favourably in historical rankings and public opinion of British prime ministers. Her tenure constituted a realignment towards neoliberal policies in Britain, with debate over the complicated legacy attributed to Thatcherism persisting into the 21st century.