Heterochromia means eyes that come in two different colors. Complete heterochromia means the color of the left eye is completely different from the color of the right eye. In cats, this most often occurs in white cats or cats that have the white-spotted gene, but can occur in any color of cat. See a collection of beautiful odd-eyed cats at Environmental Graffiti. Link
(Image credit: Flickr user P!XELTREE)