Plain English Meaning
A very small, light, one-person flying machine flown only for fun. Because it is so light and slow, the FAA does not treat it as a regular aircraft, so the pilot does not need a pilot's license and the vehicle does not need to be certified.
Full Definition
A single-occupant aeronautical vehicle, used or intended to be used for sport or recreation only, that meets specific weight, speed, and fuel limits set out in 14 CFR Part 103. It is not classified as an aircraft and does not require FAA airworthiness certification or a pilot certificate to operate.
Context Anchor
You may see this term in FAA rules, airport discussions, chart notes, or local procedures when very light sport or recreational flying machines operate near an airport.