Open Ticket is a ticket valid for a Travel between specified points without a Reservation or a specific flight number.
Other definition:
An Open Ticket is a ticket with no date specified and the passenger books a flight when ready to travel.
These are usually full fare tickets, as opposed to a discounted, restricted fare.
Other definition:
An Open Ticket is valid for transportation between certain points but indicates no specific reservation
Ticket that does not specify the date on which the service is to be performed, leaving the passenger to secure a reservation at a later date.