Erromango is the largest island in Tafea Province, the southernmost province of Vanuatu. Its highest point is Mount Santop, at 886 m. Its largest villages are Port Narvin (Potnarvin) and Dillons Bay (Upongkor). The former main village was Ipota. The total area of Erromango is 888 km².
It was formerly known as Martyr's Island by the Presbyterian missionaries in the 19th century, because of the risk attached to missionary activity there—notably the death of John Williams of the London Missionary Society at Dillon Bay in 1839. In December 2009, after a lengthy collaboration between the Museum of Anthropology at the University of British Columbia and church and cultural leaders in Vanuatu, descendants of Williams travelled to Erromango to accept the apologies of descendants of those who killed their ancestor in a ceremony of reconciliation. To mark the occasion, Dillons Bay was renamed Williams Bay.
The youngest volcanoes on the island are the three stratovolcanoes that form the Traitor's Head peninsula, north of Cook Bay on the eastern coast. A submarine vent between the peninsula and nearby Goat Island erupted in 1881.