A tower built by Finrod Felagund on a headland westward of Eglarest. Its purpose was to watch the western seas, and warn of any naval attack by Morgoth's forces. As Morgoth never mounted any attack from the ocean, the tower was never in fact needed. In the year after the Nirnaeth Arnoediad, though, Morgoth did launch a land-based attack on Eglarest and Brithombar, and Barad Nimras was torn down at that time by the very foes it was meant to guard against.
The name Barad Nimras means 'Tower of the White Horn', but it's far from clear what the 'white horn' represents. It may be a reference to the tower itself, or perhaps to the narrow cape on which it stood.