What was Mirkwood called before it became Mirkwood in Tolkien’s Middle-earth?

Before it became Mirkwood, the forest in J.R.R. Tolkien’s Middle-earth was called Greenwood the Great.

Greenwood the Great was once a vast and comparatively wholesome woodland east of the Misty Mountains. Its name changed after Sauron established the stronghold of Dol Guldur in the southern part of the forest. The shadow of his presence corrupted the region, and the woodland became known as Mirkwood, or Eryn Lasgalen in Sindarin after Sauron’s defeat.

The forest is important in both The Hobbit and The Lord of the Rings. In The Hobbit, Bilbo and the Dwarves cross it while travelling toward the Lonely Mountain, encountering giant spiders and the Elvenking’s halls. In The Lord of the Rings, Mirkwood remains a major realm of the Woodland Elves under King Thranduil.

Mirkwood is not the same forest as Fangorn or the Old Forest. Tolkien uses several distinct woodlands, each with its own history, inhabitants, and atmosphere.