Tristan da Cunha, colloquially Tristan, is both a remote group of volcanic islands in the south Atlantic Ocean and the main island of that group. It is the most remote inhabited archipelago in the world, lying 2,816 km from the nearest land, South Africa, and 3,360 km from South America. The territory consists of the main island of Tristan da Cunha itself, which has a north-south length of 11.27 kmand has an area of 98 square km along with the smaller, uninhabited Nightingale Islands and the wildlife reserves of Inaccessible and Gough Islands.

Tristan and Gough occupy an interesting position in the South Atlantic in relation to ocean currents, being at the boundary between the Southern Ocean and the South Atlantic where waters of different characteristics meet. North of the islands, the huge anticlockwise gyre of the South Atlantic brings warm water towards the islands from the direction of Brazil and Argentina, while to the south of the archipelago, cold water of the Antarctic Circumpolar Current (ACC) runs west to east right around the globe, circling the Antarctic continent. The Tristan islands are located in the meeting zone between these two major circulatory features. Strong westerly and northwesterly winds drive these currents eastwards, in the broad zone known as the West Wind Drift, which continues right across the southern Indian Ocean to Australia and New Zealand. 

Tristan da Cunha is part of the British overseas territory of Saint Helena, Ascension and Tristan da Cunha. The Subtropical Convergence, the boundary between the warm Atlantic gyre and cold water to the south, lies approximately at the Tristan top islands, with residual currents from west and northwest. Gough lies south of this boundary, so that seawater temperatures at Gough are 3-4°C lower than at the top islands, and residual currents are from the west. This results in significant differences in the marine life at Gough, which is more subantarctic in nature than at the northern islands.  Gough has its own version of a 'coral reef', a complex structure in shallow water at 0-4m, made up, not of animal coral, but of hard, leafy coralline seaweed, often overgrowing the shells of large barnacles. This complex carbonate structure is a refuge for many small animals, including lobsters. 

The islands are surrounded by underwater forests of the giant kelp, one of the fastest growing plants on the planet. Its fronds are known to grow up to 60cm (two feet) a day, and to over 50m (150 feet) in total length. The kelp forests are highly productive and support many other marine species.   The Tristan top islands appear to be a stronghold for the rarely seen Shepherd's Beaked Whale, and also a small nursery area for Southern Right Whales, which were formerly much more numerous around Tristan but were hunted intensively.   Seven endemic land birds are found on the islands, including a thrush, four 'buntings' and two flightless rails, the Inaccessible Rail being the world's smallest extant flightless bird. The buntings have undergone an adaptive radiation into small-billed and large-billed forms on the Tristan archipelago similar to Darwin's finches on the Galapagos, but the population on the main island of Tristan went extinct during the 19th century. The Gough Bunting is Critically Endangered due to predation by introduced House Mice.