a group of four small volcanic islands in the S Atlantic, about halfway between South Africa and South America: comprises the main island of Tristan and the uninhabited islands of Gough, Inaccessible, and Nightingale; discovered in 1506 by the Portuguese admiral Tristãoda Cunha; annexed to Britain in 1816; whole population of Tristan evacuated for two years after the volcanic eruption of 1961. Pop: 264 (2010 est). Area: about100 sq km (40 sq miles)