giant star

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several classes of highly luminous stars that are very massive, very large, or both.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy A star that has stopped fusing hydrogen in its core and as a result has grown much larger and brighter.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a very bright star of large diameter and low density (relative to the Sun)