From Tolkien Gateway
Westemnet was the westernmost grasslands of Rohan beyond Entwash that spread until the rivers Isen, marking the western part of the land of the Mark.
The most important of Rohan's settlements and fortifications lay to the south of the Westemnet, where the White Mountains rose up from the plain. Helm's Deep was among these, and to the south-east was the seat of the Lords of the Mark, Edoras itself.
Emnet means "plain" in Rohanese.