The brethil, a rare breed of birch tree found randomly throughout Middle-earth, was a fair and beautiful tree. The name "brethil" in Sindarin meant "silver birch" ('bre' meaning 'associated with trees' and 'thil' meaning 'to shine with a white or silver light').

With its smooth trunks, white bark, and green leaves, the brethil tree was very popular among the elves. In autumn, the leaves of this tree turned a pale yellow while the trunk remained it's normal waxy, white color. With inconspicuous growth rings, this tree was used frequently in making furniture.

Having an ancient history, the Forest of Brethil was home to the people of Haladin. The Forest of Brethil was located in the western area of Beleriand.