Hydra, or Ydra or Idra (Ýdra, [ˈiðra] , Arvanitika: Nύδρα/Nidhra), and in antiquity Hydrea, is one of the Saronic Islands of Greece, located in the Aegean Sea between the Myrtoan Sea and the Argolic Gulf. It is separated from the Peloponnese by a narrow strip of water. In ancient times, the island was known as Hydrea (Ὑδρέα, derived from the Greek word for "water"), a reference to the natural springs on the island.
The municipality of Hydra consists of the islands Hydra (pop. 1,948, area ), Dokos (pop. 18, area ), and a few uninhabited islets, total area . The province of Hydra (Επαρχία Ύδρας) was one of the provinces of the Argolis and Corinthia prefecture from 1833 to 1942, Attica prefecture from 1942 to 1964, Piraeus prefecture from 1964 to 1972 and then back to Attica as part of the newly establishment Piraeus prefecture of Attica prefecture. Its territory corresponded with that of the current municipality. It was abolished in 2006. Today the municipality of Hydra is part of Islands regional unit of Attica region.
There is one main town, known simply as "Hydra port" (pop. 1,900 in 2011). It consists of a crescent-shaped harbor, around which is centered a strand of restaurants, shops, markets, and galleries that cater to tourists and locals (Hydriots). Steep stone streets lead up and outward from the harbor area. Most of the local residences, as well as the hostelries on the island, are located on these streets. Other small villages or hamlets on the island include Mandraki (pop. 11), Kamini, Vlychos (19), Palamidas, Episkopi, and Molos. All wheeled vehicles are not permitted in the island, including but not limited, cars, motorcycles, scooters, motorbikes, bicycles,and e-bikes.
The name Hydra comes from ancient Greek ὕδρα (hydra), derived from the Greek word for "water", a reference to the natural springs on the island.
The local Arvanite name is attested in two variants: the common variant Nύδρα/Nidhra and the rare form Nidhriza or Hydriza. In Turkish it is known as Idra and in Ottoman Turkish: يدرا.