Santorini is an island located in the southern Aegean Sea, about 200 km southeast from Greece's mainland. It is also known as Thira (or Thera), forming the southernmost member of the Cyclades group of islands, with a population of 13,670 habitants. It is composed of the Municipality of Thira and the Community of Oía, which includes 268 inhabitants resident on the offshore island of Therasia, lying to the west. The total land area includes the uninhabited islands of Nea Kameni, Palaia Kameni, Aspronisi, and Christiana (all part of the Municipality of Thira).

Santorini is essentially what remains of an enormous volcanic explosion, destroying the earliest settlements on what was formerly a single island, and leading to the creation of the current geological caldera. Its spectacular physical beauty, along with a dynamic nightlife, have made the island one of Europe's tourist hotspots.

A giant central lagoon, more or less rectangular is surrounded by high steep cliffs on three sides. The island slopes downward from the cliffs to the surrounding Aegean Sea. On the fourth side, the lagoon is separated from the sea by another much smaller island called Therasia, the lagoon merges with the sea in two places, in the northwest and southwest. The water in the centre of the lagoon is nearly 400 m deep, thus making it a safe harbour for all kinds of shipping. The island's harbours all lie in the lagoon and there are no ports on the outer perimeter of the island. The capital, Fira, clings to the top of the cliff looking down on the lagoon. The volcanic rocks present from the prior eruptions feature olivine and have a notably small presence of hornblende.

It is the most active volcanic centre in the South Aegean Volcanic Arc, though what remains today is chiefly a water-filled caldera. The volcanic arc is approximately 500 km long and 20-40 km wide. The region first became volcanically active around 3-4 million years ago though volcanism on Thera began around 2 million years ago with the extrusion of dacitic lavas from vents around the region of Akrotiri. The name Santorini was given to it by the Latin empire in the thirteenth century, and is a reference to Saint Irene. Before then it was known as Kalliste ("the most beautiful one"), Strongyle ("the circular one"), or Thera.

The island is the site of one of the largest volcanic eruptions the planet has ever seen: the Minoan eruption (sometimes called the Thera eruption), which occurred some 3,600 years ago at the height of the Minoan civilization. The eruption left a large caldera surrounded by volcanic ash deposits hundreds of feet deep and may have led indirectly to the collapse of the Minoan civilization on the island of Crete, 110 km to the south, through the creation of a gigantic tsunami. Another popular theory holds that the Thera eruption is the source of the legend of Atlantis.

Santorini was united with Greece in 1912. Its major settlements include Fira (Phira), Oia, Emporio, Kamari, Perissa, Imerovigli, Pyrgos, and Therasia. Akrotiri is a major archaeological site, with ruins from the Minoan era. Santorini's primary industry is tourism, particularly in the summer months. The island's pumice quarries have been closed since 1986, in order to preserve the caldera.

Santorini has no rivers, and water is scarce. Until the early 1990s locals filled water cisterns from the rain that fell on roofs and courts, from small springs, and with imported assistance from other areas of Greece. In recent years a desalination plant has provided running, yet non-potable, water to most houses.

The island remains the home of a small, but flourishing, wine industry, based on the indigenous grape variety, Assyrtiko. Also unique to the island is the red, sweet, and extremely strong Vinsanto. White wines from the island are extremely dry with a strong, citrus scent, and the ashy volcanic soil gives the white wines a slightly sulfurous flavour much like Vinsanto. It is not easy to be a winegrower in Santorini. The hot and dry conditions give the soil a low productivity.