Mr. Zaman looks at the practice differently. “When you go to the store and buy a bag of chips, is it unethical to finish only half of it?” Mr. Zaman said. He notes that Skiplagged.com is for the average American flier — who flies two times a year, according to the trade association Airlines for America — not for someone who intends to book hidden city routes often.

“Our mission is to make it easier to experience the world,” Mr. Zaman added. “We make flying more accessible, so we’re fighting against price discrimination and the way airlines try to take advantage of everybody.”

Some airlines would beg to disagree. In the recent Lufthansa litigation, the airline sued a passenger, claiming a contract violation when he skipped the last leg of his flight on a one-stop ticket from Seattle to Oslo via Frankfurt. The airline’s conditions of carriage stipulate that a fare is only valid if flown in the booked order (the order of flights listed in the itinerary), according to a spokeswoman for Lufthansa.

Hacker fares

Many flight hacks are termed hacker fares, but Kayak popularized the term used in its search results on round-trip queries to show two one-way tickets, often on competing airlines, at rock-bottom prices.

For example, a recent search for fares from Chicago to Los Angeles turned up nonstop round trips on United from $321, versus $275 with an outbound on Delta and a return on American. Round-trip prices for the same itinerary turned up fares of $372 on Delta and $312 on American, both more expensive than that $275 combination of two one-way fares.

“Hacker fares are what Kayak does to discover cheapness, but also more diversity in results in terms of flight schedules,” Giorgos Zacharia, the chief technology officer at Kayak, said. “Domestically, if you fly out with one airline and return with a different one you can create a lot of savings.”