Sindarin: Land between the rivers
Region between the Gwathló and the Baranduin, bounded upon the east by the North-SouthRoad.
During the FirstAge and early SecondAge, Minhiriath was covered with forests (remnant of which is the ErynVorn). It was first populated by a woodland folk related to the Haladin. Númenorean mariners ravaged this land in the first milennium of the SeA, and largely deforested it; it was later further devastated by the forces of Sauron.
In the ThirdAge Minhiriath was the southernmost region of Arnor (and later Cardolan). Its chief city was Tharbad, where the North-SouthRoad crossed the Greyflood -- a city which had once been an important Númenorean port. The GreatPlague of 1636, and Cardolan's many wars, decimated the folk of this region, and the floods of the late ThirdAge finished them off. At the time of the WotR, Minhiriath was deserted, apart from a few scattered settlements and the mysterious denizens of the ErynVorn.
Minhiriath would have been repopulated and revitalised by the renewed traffic between Arnor and Gondor in the FourthAge.