brown dwarf

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

• noun A celestial body that is more massive than a gas giant planet but has insufficient mass to sustain the hydrogen fusion reactions that produce radiant energy in normal stars.

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

• noun astronomy A starlike object that contracts to about the volume of the planet Jupiter after its formation phase; its mass may range from several times that of Jupiter, such that it fuses deuterium, up to just below the threshold of sustained hydrogen fusion.