A type of electronic flash where the flash tube is shaped like a ring. The tube is generally enclosed in a ring shaped reflector with some type of diffusing plastic in the front: the whole assembly usually is the right size to be mounted in front of a lens.
The ring flash is used to make pictures of small objects (like bugs) without shadows.
It has also seen a very occasional use in fashion photography, since it gives interesting ring-shaped reflections in models' eyes.
Vivitar makes a universal ring-flash. Nikon makes a peculiar 200 mm telephoto lens with an integrated ring-flash, for some types of medical photography (forensic and dentistry scientific reporting).