A corona is a glowing circle of light around an object. You can see the Sun's corona during a total solar eclipse.
The word corona refers in a general sense to a circle or ring of light. In scientific terms, a corona is the very outer part of the Sun's gaseous atmosphere. If you want to see the corona of the Sun, you'll have to use a special viewing device during the next solar eclipse (don't look directly at the Sun — it's not safe!). In the 17th century, corona referred to a circular crown, from the Latin word for "crown or garland."