What characteristic defines the noble gases?

Noble gases are characterized by having a full octet of valence electrons in their outermost shell. This complete electron configuration contributes to their chemical stability and lack of reactivity, which is why they are termed "noble." Elements such as helium, neon, argon, krypton, xenon, and radon resonate with this attribute, making them less inclined to participate in chemical reactions compared to other elements that do not have a satisfying electron arrangement.

The full octet allows these gases to exist in their natural state without forming bonds with other elements, resulting in their reputation for being inert. This unique property sets noble gases apart, making them essential in various applications, such as lighting and welding, and as inert environments for sensitive chemical reactions.