an immense and extremely luminous star, as Betelgeuse or Antares, that has a diameter at least 100 times that of the sun and that is 100 to more than 10,000 times as bright
See supergiant in American Heritage Dictionary 4
noun
Any of various very large bright stars, such as Betelgeuse or Rigel, having a luminosity that is thousands of times greater than that of the sun.