What is "ithilien"

In J. R. R. Tolkien's fictional Middle-earth, Ithilien is a region and fiefdom of Gondor.

Ithilien, or "Moon-land," is the easternmost province of Gondor, the only part of Gondor across the Great River Anduin lying between the river and the Mountains of Shadow ( Ephel Dúath), subdivided by the stream of Morgulduin into North and South Ithilien.