9/11 refers to the terrorist attacks that occurred on September 11, 2001, when hijackers took control of four commercial airplanes, crashing two into the Twin Towers of the World Trade Center in New York City and one into the Pentagon; the fourth plane crashed into a field in Pennsylvania after passengers attempted to overcome the hijackers. This event significantly impacted global politics, leading to changes in security policies and international relations, especially within democracies.