Plain English Meaning
An aircraft that has nothing to do with whatever is happening inside a special-use airspace. If you're not part of the operation going on in there, you're a nonparticipating aircraft, and you usually have to stay out.
Full Definition
An aircraft that is not part of the activities being conducted within a designated area of airspace, particularly a Restricted Area. Nonparticipating aircraft are generally prohibited from entering an active Restricted Area because the hazardous activity inside (such as artillery firing, aerial gunnery, or guided missile operations) is not coordinated with them and could endanger them.
Context Anchor
Seen in special use airspace discussions, especially restricted areas, where certain military or hazardous activities may be taking place.
Derivation
From 'non-' (not) plus 'participating' (taking part in). The term simply identifies any aircraft not taking part in the activity inside the airspace — a plain-language label, but one with in .