Ios or Io or Nio (Greek: Ίος, Greek pronunciation: [iːos], locally Nios Greek: Νιός) is a Greek island in the Cyclades group in the Aegean Sea. Ios is a hilly island with cliffs down to the sea on most sides, situated halfway between Naxos and Santorini. It is about 18 kilometres (11 miles) long and 10 kilometres (6 miles) wide, with an area of 109.024 square kilometres (42.094 sq mi). Population was 2,024 in 2011 (down from 3,500 in the 19th century). Ios is part of the Thira regional unit.