Noble Gas
A noble gas is a group 18 element (helium, neon, argon, krypton, xenon, radon) with a full outer electron shell, making it almost completely unreactive.
Definition
Noble gases are the elements in the last column of the periodic table, and they almost never react with anything, so they are safe and stable. The six are helium, neon, argon, krypton, xenon, and radon in group $18$ (oganesson is also placed there); each has a completely full outer energy level ($2$ electrons for helium, $8$ for the rest), so they have no tendency to gain, lose, or share electrons, exist as single atoms, are colorless and odorless, and have very low boiling points. Their closed-shell $ns^2 np^6$ configurations (or $1s^2$ for helium) give them the highest ionization energies in their periods and essentially zero electron affinity. Their only interatomic attractions are weak London dispersion forces, so boiling points rise down the group with increasing polarizability, from helium ($4\text{ K}$) to radon ($211\text{ K}$), and the heavier noble gases can be forced into compounds with the most electronegative elements.
Example
Helium fills party balloons and will not burn, neon glows orange-red in signs, and argon is used inside light bulbs to protect the filament. Argon makes up about $0.93\%$ of air, more than carbon dioxide, and shields hot metal during welding because it will not react with it, while helium boils at $-269^\circ\text{C}$, the lowest of any element. Neil Bartlett prepared the first noble gas compound in 1962 by reacting xenon with platinum hexafluoride, and $XeF_2$, $XeF_4$, and $XeF_6$ soon followed; xenon's first ionization energy ($1170\text{ kJ/mol}$) is low enough that fluorine can pull electron density from it, whereas helium and neon form no stable compounds at all.
Key Insight
The Hindenburg airship burned because it was filled with hydrogen; helium would have been safe, because noble gases do not react. Other atoms bond in order to reach the electron arrangement a noble gas already has, so the octet rule is really "act like a noble gas." Ramsay and Rayleigh discovered argon in 1894 because nitrogen extracted from air was slightly denser than nitrogen made chemically, and a discrepancy in the third decimal place led to an entire new column of the periodic table.