Euboea (Greek: Εύβοια, Évia; Ancient Greek: Εὔβοια 'Eúboia') is the second largest Greek island in area and population, after Crete. It is separated from the mainland of Greece by the narrow Euripus Strait. In general outline it is a long and narrow, seahorse-shaped island; it is about 150 kilometres (93 mi) long, and varies in breadth from 50 kilometres (31 mi) to 6 kilometres (3.7 mi). Its general direction is from northwest to southeast, and it is traversed throughout its length by a mountain range, which forms part of the chain that bounds Thessaly on the east, and is continued south of Euboea in the lofty islands of Andros, Tinos and Mykonos.

Geography
Satellite picture showing central and south-eastern Euboea at the top (Attica and Boeotia in the lower portion). Orientation: north-east uppermost.
Beach at Chiliadou in Euboea.

Like most of the Greek islands, Euboea was originally known under other names in ancient times, such as Macris and Doliche from its shape, Ellopia and Abantis from the tribes inhabiting it.

Euboea was believed to have originally formed part of the mainland, and to have been separated from it by an earthquake. This is fairly probable, because it lies in the neighbourhood of a fault line, and both Thucydides and Strabo write that the northern part of the island had been shaken at different periods. In the neighbourhood of Chalcis, both to the north and the south, the bays are so confined as to make plausible the story of Agamemnon's fleet having been detained there by contrary winds. At Chalcis itself, where the strait is narrowest at only 40 m, it is called the Euripus Strait. The extraordinary changes of tide which take place in this passage have been a subject of note since classical times. At one moment the current runs like a river in one direction, and shortly afterwards with equal velocity in the other. A bridge was first constructed here in the twenty-first year of the Peloponnesian War (410 BC). The name Euripus developed during the Middle Ages into Evripo and Egripo, and in this latter form transferred to the whole island. Later the Venetians, when they occupied the district, altered it to Negroponte, referring to the bridge which connected it with the mainland.

The main mountains include Dirphys (1,745 m), Pyxaria (1,341 m) in the northeast and Ochi (1,394). The neighboring gulfs are the Pagasetic Gulf in the north, Maliakos Gulf, Northern Euboean Gulf in the west, the Euboic Sea and the Petalion Gulf. At the 2001 census the island had a population of 198,130, and a total land area of 3,684.848 km².