The U.S. Federal Aviation Administration (FAA) in the CFR Title 14 part 91 defines a Part 91 Operator. These are the regulations that define the operation of small non-commercial aircraft within the US, however, many other countries defer to these rules. These rules set conditions, such as weather, under which the aircraft may operate.
Part 135 operator
Part 135 operator rules govern commercial aircraft. FAR (Federal Aviation Regulations) Part 135 which is titled "Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons On Board Such Aircraft" applies to turbojet engine powered aircraft with 1-30 seats, non-transport category turbo-propeller powered aircraft with 10-19 seats, and transport category turbo props with 20-30 seats. Applicants for a FAR Part 135 certificate must have exclusive use of at least one aircraft.