The point of no return is the point beyond which someone, or some group of people, must continue on their current course of action, either because turning back is physically impossible, or because to do so would be prohibitively expensive or dangerous. It is also used when the distance or effort required to get back would be greater than the remainder of the journey or task as yet undertaken.
A particular irreversible action (e.g., setting off an explosion or signing a contract) can be a point of no return, but the point of no return can also be a calculated point during a continuous action (such as in aviation).
The term PNR—"point of no return," more often referred to by pilots as the "Radius of Action formula"—originated, according to the Oxford English Dictionary, as a technical term in air navigation to refer to the point on a flight at which, due to fuel consumption, a plane is no longer capable of returning to its airfield of origin. After passing the point of no return, the plane has no option but to continue to some other destination. In this sense, the phrase implies an irrevocable commitment. The PNR is, for nonstop flights, actually beyond the halfway (more exactly, the "equitime") point, since airplanes carry spare fuel, and since later in a flight the aircraft uses less fuel (it is lighter because it is carrying less fuel). For example, on a 2000-mile flight, should the tanks have enough fuel for a 3000-mile flight, the halfway point would be at 1000 miles, but the PNR would be at more than 1500 miles.
Another aviation use is the point during the takeoff roll when there is no longer enough runway ahead of the airplane to safely stop; at this point, the aircraft is committed to taking off.
In mountain aviation, the phrase is sometimes used in a completely different way to refer to the point at which the grade of the terrain "outclimbs" the aircraft—that is, the point at which a crash is inevitable, being a parallel in common usage. The phrase can also be used in this sense to denote inevitable disaster.
In relation to black holes, the point of no return refers to the point at which nothing can escape the supreme gravitational pull of the black hole. This point is more commonly known as the event horizon.