Plain English Meaning
The rules that say an aircraft is safe and legal to fly. The aircraft must match how it was originally approved, be in good working condition, and have all its required inspections and paperwork up to date.
Full Definition
The set of conditions an aircraft must meet to be considered legally and mechanically safe for flight, as defined by the FAA. This includes conforming to its approved type design, being in a condition for safe operation, and having all required inspections, maintenance actions, and Airworthiness Directives complied with and properly documented.
Context Anchor
You will meet this term during preflight planning, aircraft inspections, maintenance record checks, and any decision about whether an is legal and safe to fly.