From AMS Glossary
(Also called globe lightning.) A rare and randomly occurring bright ball of light observed floating or moving through the atmosphere close to the ground.
Observations have widely varying identifying characteristics for ball lightning, but the most common description is that of a sphere having a radius of 15–50 cm, orange or reddish in color, and lasting for only a few seconds before disappearing, sometimes with a loud noise. Most often ball lightning is seen in the vicinity of thunderstorms or a recent lightning strike, which may suggest that ball lightning is electrical in composition or origin. Considered controversial due to the lack of unambiguous physical evidence for its existence, ball lightning is becoming more accepted due to recent laboratory recreations resembling ball lightning. Despite the observations and models of these fire balls, the exact mechanism(s) for naturally occurring ball lightning is unknown.