American Airlines Flight 77 was a scheduled domestic transcontinental passenger flight from Dulles International Airport in Northern Virginia to Los Angeles International Airport in Los Angeles. The Boeing 757-223 aircraft serving the flight was hijacked by five al-Qaeda terrorists on the morning of September 11, 2001, as part of the September 11 attacks. The hijacked airliner was deliberately crashed into the Pentagon in Arlington County, Virginia, killing all 64 aboard and another 125 in the building.

Aircraft hijacking is the unlawful seizure of an aircraft by an individual or a group. Dating from the earliest of hijackings, most cases involve the pilot being forced to fly according to the hijacker's demands. There have also been incidents where the hijackers have overpowered the flight crew, made unauthorized entry into the cockpit and flown them into buildings – most notably in the September 11 attacks – and in several cases, planes have been hijacked by the official pilot or co-pilot; e.g., Ethiopian Airlines Flight 702.