In Tolkien'S Legendarium, the Eye of Sauron is both the symbol of Sauron worn by his soldiers, and a metaphysical image of his projected will which could be sensed by Frodo Baggins. The Eye is described as being yellow with a slit pupil, lidless and rimmed with fire with a piercing gaze.
In Peter Jackson's film adaption of The Lord Of The Rings trilogy, the Eye is depicted as a literal flaming eye atop Barad-dûr.