Noble Gas
Definition
The group 18 elements: helium, neon, argon, and friends. They almost never react because their outer electron shells are completely full.
Technical definition
Noble gases are the group 18 elements (He, Ne, Ar, Kr, Xe, Rn) with filled valence shells (ns^2 np^6, except helium at 1s^2). Their high ionization energies and near-zero electron affinities make them the least reactive elements, used wherever inert atmospheres are needed.
Example
Argon fills incandescent light bulbs and welders' torches so hot metal does not react with air. Helium's low density lifts balloons.
Common confusion
Inert does not mean absolutely unreactive. Krypton and xenon can form compounds with fluorine and oxygen under extreme conditions, such as XeF4, though helium, neon, and argon remain stubbornly aloof.