Hint: a regulation exists but it is not in part 61 or 91 (think, space shuttle)...
This AC describes a fair bit of what/where a traffic pattern is. ultimately it comes down to definition but the answer seems to be what ever the defined traffic pattern says
The airport traffic patterns described in part 93 relate primarily to those airports where there is a need for unique traffic pattern procedures not provided for in part 91. Part 97 addresses instrument approach procedures (IAP). At airports without operating control towers, part 91 requires only that pilots of airplanes approaching to land make all turns to the left, unless light signals or visual markings indicate that turns should be made to the right
It seems they are somewhat careful to not explicitly define anything and instead allow some definitions to define it for explicit cases and others to be left to interpretations. The FAA advises that calls be made as far as 10 miles out.
The FAA technically allows airports to define their traffic pattern themselves:
Traffic Pattern Design. Airport owners and operators, in coordination with the FAA, are responsible for establishing traffic patterns. The FAA encourages airport owners and operators to establish traffic patterns as recommended in this AC. Further, left traffic patterns should be established, except where obstacles, terrain, and noise-sensitive areas dictate otherwise
They do provide some guidance on how to do so in regards to altitude they even allow the pilot to decide
Traffic Pattern Altitudes. It is recommended that airplanes observe a 1,000 foot above ground level (AGL) traffic pattern altitude. Large and turbine-powered airplanes should enter the traffic pattern at an altitude of 1,500 feet AGL or 500 feet above the established pattern altitude. Ultralight aircraft should operate no higher than 500 feet below the powered aircraft pattern altitude. A pilot may vary the size of the traffic pattern depending on the aircraft’s performance characteristics
Going straight to regulations taken from here
§91.127 Operating on or in the vicinity of an airport in Class E airspace.
(b) Departures. Each pilot of an aircraft must comply with any traffic patterns established for that airport in part 93 of this chapter.
and for class D
§91.129 Operations in Class D airspace.
(1) Unless required by the applicable distance-from-cloud criteria, each pilot operating a large or turbine-powered airplane must enter the traffic pattern at an altitude of at least 1,500 feet above the elevation of the airport and maintain at least 1,500 feet until further descent is required for a safe landing.
and Class C
§91.130 Operations in Class C airspace.
(b) Traffic patterns. No person may take off or land an aircraft at a satellite airport within a Class C airspace area except in compliance with FAA arrival and departure traffic patterns.
If the tower is in operation they can vector you as they see fit (at least here under the FAA) as such they may use pattern terms and organization but I cant tell if the FAA considers it a pattern or not if there is a tower.
FAR 157.2 for airport planning states, "Traffic pattern means the traffic flow that is prescribed for aircraft landing or taking off from an airport, including departure and arrival procedures utilized within a 5-mile radius of the airport for ingress, egress, and noise abatement."
In other words, once you are within the airport operation area, you are part of the "traffic pattern". The airport operation area is 5 statute miles unless otherwise noted and therefore makes no difference whether a control tower is operating.
