Literally, a new star. An existing star which suddenly increases its brightness by more than 10 magnitudes and then slowly fades. This happens when a star throws off a portion of its outer shell or envelope; after this happens, the star usually returns to normal. On the average, 25 such eruptions per year occur in a spiral galaxy such as our own (the Milky Way). But, only 2 or 3 new novae are usually discovered in our galaxy each year.