If an organization such as an airline or a theatre company overbooks, they sell more tickets than they have places for. Planes are crowded, airlines overbook, and departures are almost never on time. to sell more tickets for a theatre, plane etc than there are seats available
If something such as a hotel, bus, or aircraft is overbooked, more people have booked than the number of places that are available. Sorry, the plane is overbooked
The practice of confirming more seats on an aircraft or rooms in a hotel than actually are available Overbooking is designed to protect the airline or hotel from no-shows
The practice of taking more reservations than there are seats, rooms, or space in the expectation that no shows (qv) will bring the number of reservations actually used below maximum occupancy