Throughout The Lord of the Rings, "the Eye" (the Red Eye, the Evil Eye, the Lidless Eye, the Great Eye) is the image most often associated with Sauron. Sauron's Orcs bore the symbol of the Eye on their helmets and shields, and referred to him as the "Eye" because he did not allow his name to be written or spoken, according to Aragorn[ (a notable exception to this rule was his emissary, the Mouth of Sauron). Also, the Lord of the Nazgûl threatened Éowyn with torture before the "Lidless Eye" at the Battle of the Pelennor Fields.
Frodo had a vision of the Eye in the Mirror of Galadriel:
The Eye was rimmed with fire, but was itself glazed, yellow as a cat's, watchful and intent, and the black slit of its pupil opened on a pit, a window into nothing.
Later, Tolkien writes as if Frodo and Sam really glimpse the Eye directly. The mists surrounding Barad-dûr are briefly withdrawn, and:
one moment only it stared out...as from some great window immeasurably high there stabbed northward a flame of red, the flicker of a piercing Eye... The Eye was not turned on them, it was gazing north...but Frodo at that dreadful glimpse fell as one stricken mortally.
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