noble gas

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of the elements in Group O of the periodic table, including helium, neon, argon, krypton, xenon, and radon, which are monatomic and with limited exceptions chemically inert. Also called inert gas.

Wiktionary

1. n. chemistry Any of the elements of group 18 of the periodic table, being monatomic and (with very limited exceptions) inert.

GNU Webster's 1913

1. adj. (Chem.) a gaseous element belonging to group VIII of the periodic table of elements, not combining with other elements under normal reaction conditions; specifically, helium, neon, argon, krypton, xenon, or radon; also called inert gas.

WordNet 3.0

1. n. any of the chemically inert gaseous elements of the helium group in the periodic table