The point of no return is the point beyond which, in any form of travel, you don't have the option of turning back any more, because you would not have the resources (fuel, time ...) to make it back to the starting point.

In a sense, once you go beyon the point of no return, you are committed to reaching your destination, or being stuck somewhere in the middle.

It is mostly used a navigation/military concept, in an aeronautical or naval concept.
It is also closely related to the radius of action figure for a given vehicle.
Due to varying efficiency, the point of no return may change depending on the speed at which the vehicle travels.

NB: it is not necessarily the midpoint of a trip, and it may shift as environment conditions change.