brown dwarf

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A celestial body with insufficient mass to sustain the nuclear fusion that produces radiant energy in normal stars, believed to have formed with enough mass to start nuclear fusion in its core, but without enough for the fusion to become self-sustaining.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. 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.