A 20th hijacker is a numeric metaphor concerning a possible additional terrorist in the September 11, 2001 attacks who was not able to participate.
The term is somewhat misleading, as there is no evidence that al-Qaeda ever planned to have exactly 20 hijackers. 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, thus the idea of a 20th hijacker came to be widely discussed....
The 9/11 Commission concluded that eight members of al-Qaeda, in addition to the 19 hijackers, attempted to enter the United States to participate in the attacks.
Ramzi Binalshibh was repeatedly denied entry into the U.S and was unable to take part. Mohamed al-Kahtani was another would-be hijacker, but he was denied entry into the U.S. at Orlando International Airport in August, 2001. He was later captured and imprisoned at Guantanamo Bay.
Zacarias Moussaoui was considered as a replacement for Ziad Jarrah, who at one point threatened to withdraw from the scheme because of tensions amongst the plotters. Plans to include Moussaoui were never finalized, as the al-Qaeda hierarchy had doubts about his reliability. Ultimately, Moussaoui did not play a role in the hijacking scheme. He was arrested about four weeks before the attacks.
The other al-Qaeda members who attempted to take part in the attacks, but were not able, were Saeed al-Ghamdi (not to be confused with the successful hijacker of the same name), Mushabib al-Hamlan, Zakariyah Essabar, Ali Abdul Aziz Ali and Tawfiq bin Attash. Khalid Sheikh Mohammed, the attack's mastermind, had wanted to remove at least one member — Khalid al-Mihdhar — from the operation, but he was overruled by Osama bin Laden.
According to the BBC(http://news.bbc.co.uk/1/hi/world/middle_east/5101954.stm), it has been announced that the 20th hijacker was "Fawaz al-Nashimi". An Al-Qaeda video has been released from a US intelligence organisation, showing al-Nashimi justifying attacks on the west.[More....]