A rather nasty birth defect in which the child is born with only one oversized eye in the center of their forehead, where the root of the nose would usually be. The nose itself is either missing completely or takes the form of a tube-like organ, called a proboscis, placed above the single eye. This eye, the distinguishing feature of the disorder from which the name was drawn(see: cyclops), can either take the form of a single large eyeball or two eyeballs in the same socket - alternatively, the two eyeballs can be fused together to make one mis-shapen and presumably useless mass.

The abnormal facial features actually stem from deeper developmental problems involving the brain - children with cyclopia invariably also have holoprosencephaly, a failure of the forebrain to seperate properly into two seperate hemispheres. As the shape of the forebrain is used as a sort of reference point for facial development, fused cerebral hemispheres lead inevitably to severe facial deformities, like this one.

Strangely, this disease is caused by a defect in a gene called sonic hedgehog. I can't even begin to imagine what was going through the researcher's heads when they decided to name a gene that encodes for a morphogen, a class of signalling proteins essential to embryonic development, after a videogame character. What's next, cancer treatment therapies named after everyone's favourite plumber?

For further info, Medterms...
...Has a slightly better write-up to this, with links to associated disorders. And remember: google is your friend.