Bumping

Bumping refers to the situation in which a passenger with a valid ticket and reservation is denied boarding because the airline has sold more tickets than there are available seats. It can also happen due to an aircraft change or operational disruption. 

Some airlines intentionally overbook flights because they know that a percentage of passengers typically cancel or fail to show up.

Airlines usually ask for volunteers willing to give up their seats in exchange for compensation such as vouchers, future upgrades, or cash.

If too few volunteers step forward, the airline selects passengers. Airlines must provide bumped flight compensation for passengers who are bumped involuntarily.