Red eye
From The Glossary of Digital Photography
The condition of a subject's eyes appearing to be red. The irises of the eyes appear red instead of black because light from the flash is reflected back from the inside of the retina of the eye directly to the camera. This problem can be avoided by moving the flash head away from the center of the lens, either to one side or higher on an arm. This action is difficult to perform when hot shoe flashes don't have special
adapters or cables.
