20th hijacker is a possible additional terrorist in the September 11, 2001, attacks who was not able to participate.
There were many variations of the 9/11 plot, with the number of terrorists fluctuating with available resources and changing circumstances. In the end, there were 19 hijackers: three of the planes were taken over by five members each and the fourth was hijacked by only four people. One plane, United Airlines Flight 93, had fewer hijackers than the rest and was also less successful in its mission—instead of hitting any of the obvious targets in Washington, D.C., it hit a hillside in Pennsylvania, due to resistance from passengers. Thus the idea of a 20th hijacker came to be widely discussed.