A one-way flight is booked from the departure airport to the destination airport with no scheduled return.
There are a number of reasons to purchase a one-way flight ticket, including:
Generally it’s cheaper to buy a roundtrip, but there are several exceptions.
As a rule of thumb, a roundtrip domestic ticket typically costs the same as two one-way domestic fares. For international travel, however, it is often much cheaper to purchase roundtrip fares rather than individual one-way flights.
Additionally, low-cost or budget carriers are typically more likely to price one-way fares at half the cost of a roundtrip than their legacy airline counterparts. This is due to several reasons, including route models (budget carriers usually employ a point-to-point system while legacy airlines operate more complicated hub-and-spoke routes), price discrimination based on types of travelers, and the tendency for budget carriers to use simplified pricing (i.e. a roundtrip fare on a low-cost airline is more likely to be exact sum of each of its legs as opposed to a legacy roundtrip fare, which may be built using a more complicated algorithm).
Some airlines like Southwest and JetBlue are known for their one-way flash sales. During these special events, passengers can purchase tickets for one-way flights at a very low price. These are usually offered for a list of set routes and overall savings can vary by departure and destination airport. Therefore, it might make sense for a vacation traveler to snatch up a $39 one-way fare from New York JFK to San Juan, Puerto Rico during a flash sale and search for a return one-way ticket later.
If a roundtrip fare is a better deal than a single one-way ticket, you can purchase a roundtrip fare and cancel (or simply not board) the return flight. This is also known as a “throwaway ticket” and employs a similar strategy as hidden city ticketing.
Travelers interested in buying a throwaway ticket should note that while it’s not technically illegal, airlines frown upon this practice and can penalize passengers who frequently miss scheduled connections or return flights by invalidating frequent flyer miles, not granting or honoring airline status, or even going so far as to ban them from future travel with that airline. Also, beware that once you skip a leg, the remaining itinerary will be canceled.