means A star that suddenly brightens enormously for a time before fading back, caused by a thermonuclear flare-up on its surface — not the star's actual destruction.

from Straight from Latin nova, the feminine of novus, "new" — short for the astronomer's phrase stella nova, "new star," since these objects seemed to appear out of nowhere in the sky. The Latin novus is an old, deep word, a cousin of Greek neos and English new. So the irony is built into the name: a nova is hailed as "new" precisely at the moment an old star is flaring up.