Definition from Nolo’s Plain-English Law Dictionary
A hotel or rental car reservation secured by a credit card number. In exchange for your card number, the hotel or rental agency promises to have a room or vehicle for you no matter when you show up. If you have a guaranteed reservation with a hotel, it must provide you with a room, either at that hotel or at another comparable establishment. If you have a guaranteed reservation with a car agency, it must provide you with a vehicle. The downside of a guaranteed reservation is that if you don't show up and haven't cancelled your reservation, you will be billed for one night in the room or one day's use of the vehicle.