Ios, (also spelled Io or Nio; Greek: Ίος, Greek pronunciation: [ˈi.os]; Ancient Greek: Ἴος [í.os]; locally Nios, Νιός) is a Greek island in the Cyclades group in the Aegean Sea. The island is largely hilly island with cliffs down to the sea on most sides. It is situated halfway between Naxos and Santorini. Ios measures about 18 kilometres (11 miles) in length and 10 kilometres (6 miles) in width, and covers an area of 109.024 square kilometres (42.094 sq mi). Its population was 2,299 in the 2021 census (down from 3,500 in the 20th century). Ios is part of the Thira regional unit.