1. Form of radiant energy that acts upon the retina of the eye, optic nerve, etc., making sight possible. This energy is transmitted at a velocity of about 186,000 miles per second by wavelike or vibrational motion.
2. A form of radiant energy similar to this, but not acting on the normal retina, such as ultraviolet and infrared radiation.
Source: NASA (http://earthobservatory.nasa.gov/Glossary)