The term corona indicates glowing plasma surrounding a star. The term was first used for the glow seen around the Sun during solar eclipses. The corona of the Sun is "active", changing visibly over time. It is considered a portion of the Sun's atmosphere, external to the chromosphere, with a thin transition region between. Its temperature is much higher than the surface of the Sun.
The word is also incorporated in terms for glows surrounding other astronomical objects, such as galactic corona.
Coronagraphs were first developed to attempt to study the Sun's corona at other times than eclipses, but the term has come to mean any telescope feature designed to block out the light of the Sun or a star in order to see radiation from near the star that is much less intense.