A point at which the player may not turn back. You may not revisit any prior locations or start any missed sidequests; you MUST continue your journey until you
beat the game, and any content you avoided up to this point may not be reached.
The Point of No Return is always preceded
immediately by the Point of Safe Return, which is the last chance to do sidequests or stock up for the journey. Similar Plot Tunnels lock the player onto a set course for a while, but they don’t lead straight to the end of the game. Game Levels are where you’re frequently prevented from returning to previous areas.
The term “Point of No Return” originates in aviation, where a plane that crosses this point must continue toward its destination.
Point of No Return, Plot Tunnel, and Game Level all have pages on TV Tropes, where you can read more about them and what games they’re in.