Plain English Meaning
A balloon with nobody on it that floats wherever the wind takes it. Because nobody is steering it, the FAA has rules about where, when, and how it can be released.
Full Definition
A balloon that carries no person on board and drifts freely with the wind, without any means of onboard control or propulsion. Under FAA rules, unmanned free balloons are subject to specific operating limits regarding payload weight, launch conditions, marking, lighting, and proximity to people, property, and aircraft.
Context Anchor
Seen in FAA operating rules for balloons that may enter airspace used by aircraft, such as weather or research balloons.