Which characteristic defines inert gases?

Inert gases are characterized by their inability to support combustion, which is due to their full valence electron shells. This stability results in a lack of reactivity with other elements and compounds under standard conditions. Therefore, they do not participate in chemical reactions that involve fire or combustion.

This property makes inert gases, such as helium, neon, argon, xenon, and krypton, useful in various applications, including providing an unreactive atmosphere in laboratories and industries, preserving materials, and enhancing safety in situations where combustion could pose a risk.

The other characteristics mentioned, such as supporting combustion, reacting vigorously, or being highly flammable, are not applicable to inert gases, as these gases are specifically defined by their lack of reactivity.