Margaret Hilda Thatcher, Baroness Thatcher, LG, OM, DStJ, PC, FRS, HonFRSC (née Roberts; 13 October 1925 – 8 April 2013), was a British stateswoman and Conservative politician who served as Prime Minister of the United Kingdom from 1979 to 1990 and Leader of the Conservative Party from 1975 to 1990. She was the first female British prime minister and the longest-serving of the 20th century. As prime minister, she implemented economic 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
TL;DR: Reforms and a stern character.
Source: https://en.wikipedia.org/wiki/Margaret_Thatcher