The airline-industry term for selling more tickets than available seats is overbooking.
Airlines overbook some flights because historical data shows that a portion of passengers who reserve seats will not travel. The practice aims to keep aircraft cabins fuller and reduce the financial effect of no-shows.
If more passengers arrive than there are seats, the airline may first ask for volunteers to take a later flight, often offering compensation or travel benefits. If volunteers are insufficient, passengers can be denied boarding under the applicable laws and airline conditions of carriage.
Overbooking is not the same as overselling an entire route or changing a flight’s schedule. It refers specifically to accepting reservations beyond the aircraft’s available capacity for a particular departure.