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Point beyond which turning back is no longer possible
The point of no return (PNR or PONR) is the point beyond which one must continue on one's current course of action because turning back is no longer possible, being too dangerous, physically difficult, or prohibitively expensive to be undertaken. The point of no return can be a calculated point during a continuous action (such as in aviation). A particular irreversible action (such as setting off an explosion or signing a contract) can be a point of no return.

The phrase "point of no return" originated as a technical term in air navigation to refer to the time and/or location during a flight at which the aircraft no longer has enough fuel to return to its originating airfield. Important decisions may need to be made prior to the point of no return, since it will be unsafe to turn around and fly back if the pilot changes their mind after that point. Otherwise, it may correspond to the aircraft's maximal safe range in a situation where the only possible landing site is the takeoff site, for example in the case of an aircraft to an aircraft carrier that is underway and distant from any airfield. In those conditions, an aircraft must always have enough fuel for a return flight, so the "point of no return" may represent the point before which the pilot must return or else risk catastrophe. It can also mean the instance in which an aircraft taxis down a runway, gaining a certain speed, and must become airborne in lieu of a crash or explosion on the runway (V1 speed)—for example, Charles Lindbergh's takeoff in The Spirit of St. Louis in 1927 in which there was uncertainty about the plane's ability to take off from a 5,000 foot mud soaked runway while fully loaded with aviation fuel. The first major metaphorical use of the term in popular culture was in the 1947 novel Point of No Return by John P. Marquand. It inspired a 1951 Broadway play of the same name by Paul Osborn. The novel and play concern a pivotal period in the life of a New York City banker. In the course of the story, the character faces two "point of no return" realities: first, that his quest for a big promotion will mean either triumph or a dead end to his career, and second, that he can never go back to the small town life he abandoned as a young man.