Ithilien is a region and fiefdom of Gondor.
Ithilien, or Moon-land, is the only part of Gondor across the Great River Anduin, wedged in between the river and the Ephel Dúath of Mordor. The region is further divided into North and South Ithilien.
It was a fair and prosperous land during the Second Age and the first part of the Third Age, when Gondor was strong and Mordor deserted. Of old its chief city was Minas Ithil, but when this was captured by Mordor it was renamed Minas Morgul. After this the majority of its people fled across the Anduin to escape war, but the Stewards of Gondor still kept scouts in Ithilien, based on secret locations such as Henneth Annûn.
In The Lord of the Rings, Gollum leads Frodo Baggins and Samwise Gamgee through Ithilien on the way to the pass of Cirith Ungol into Mordor. After witnessing a battle with Southrons of Harad accompanied by Oliphaunts, the hobbits are found by Faramir, the son of the Steward Denethor, but are allowed to continue when he is satisfied they are not agents of Sauron.
During the Fourth Age, Ithilien is ruled by the Princes of Ithilien, a line that starts with Faramir and Éowyn (the White Lady of Ithilien). Minas Morgul is not repopulated, and Faramir rules as Lord of the Emyn Arnen. After the fall of Sauron, Legolas brought elves from Greenwood, and they dwelt in Ithilien, and once again it became the fairest country in all the westlands.