To overbook is to reserve seats or rooms for more people than there are seats or rooms available.
An example of overbook is to sell more tickets than there are seats on a flight.
to issue more reservations for (an airline flight, hotel, etc.) than there are accommodations
verbo·ver·booked, o·ver·book·ing, o·ver·books
To take reservations for (an airline flight, for example) beyond the capacity for accommodation.
To take reservations beyond the capacity for accommodation: a restaurant that regularly overbooks for dinner.