What was Mirkwood called before it became the dark forest in J.R.R. Tolkien’s legendarium?

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

Greenwood the Great was a vast woodland east of the Anduin. Over time, the influence of the Necromancer—later revealed to be Sauron—spread through its southern region. The forest then became known as Mirkwood, reflecting its growing danger, darkness, and corruption.

Mirkwood is especially important in The Hobbit, where Bilbo and the Dwarves cross it on their way toward the Lonely Mountain. It also appears in the wider history of Middle-earth, including the realm ruled by Thranduil. Mirkwood is not the same as Fangorn Forest, which lies much farther west and is associated with the Ent Treebeard.