Plain English Meaning
A plane that flies over an area to watch for other aircraft and make sure it is safe before any dangerous activity, like firing weapons, takes place.
Full Definition
An aircraft used to observe a designated area from the air and confirm it is clear of nontaking-part traffic before, and during, hazardous activities such as live-fire exercises in a Controlled Firing Area (CFA).
Context Anchor
Seen in Controlled Firing Area discussions, where firing or similar hazardous activity is allowed only while the area is kept clear of other aircraft.
Derivation
From 'spot,' meaning to see or notice, plus 'aircraft.' A spotter is someone whose job is to watch carefully for something specific. The aviation usage keeps that everyday meaning: an aircraft whose role is to look out for other traffic.
Why Pilots Care
Entering an active area risks collision with artillery or other fire; the spotter aircraft helps maintain the safety that keeps pilots out of danger.