A frequent-flyer program (FFP) is a loyalty program offered by an airline. Many airlines have frequent-flyer programs designed to encourage airline customers enrolled in the program to accumulate points (also called miles, kilometers, or segments) which may then be redeemed for air travel or other rewards. Points earned under FFPs may be based on the class of fare, distance flown on that airline or its partners, or the amount paid. There are other ways to earn points. For example, in recent years, more points have been earned by using co-branded credit and debit cards than by air travel. Another way to earn points is spending money at associated retail outlets, car hire companies, hotels, or other associated businesses. Points can be redeemed for air travel, other goods or services, or for increased benefits, such as travel class upgrades, airport lounge access, fast track access, or priority bookings. Frequent-flyer programs can be seen as a certain type of virtual currency, one with unidirectional flow of money to purchase points, but no exchange back into money.