The Eye: what was it?

[ElanorFB]

I've always thought of the eye as being a spiritual (for lack of a better term) manifestation. Sort of operating on a slightly different plane. Frodo is thus able to see it/ feel it more clearly due to the ring. Others may feel a sense of dread when under its gaze/control. While it was maybe a little big in the movie, I thought it operated fine as an image.