An ocean liner is a passenger ship or passenger-cargo ship that transports people and often freight from one port to another along regular trans-oceanic routes according to a schedule. The term also refers to vessels designed to engage in such trades, even if temporarily used for other purposes (such as on cruises or as troopships). The category does not include ferries or other vessels engaged in short-sea trading, nor cruise ships where the voyage itself, and not transportation, is the prime purpose of the trip. Nor does it include tramp steamers even if equipped to handle limited numbers of passengers, nor other cargo vessels (although many shipping companies refer to themselves as "lines" and their container ships, which often operate over set routes according to established schedules, as "liners"). Ocean liners typically were strongly built with high freeboards to withstand sea states and adverse conditions encountered in the open ocean, and had large capacities for fuel and other stores which would be consumed on their multi-day or multi-week voyages.

Ocean liners were the primary mode of intercontinental travel for over a century, from the mid-19th century to the late 1960s, when they were finally supplanted by airliners. In addition to passengers, liners also carried mail and cargo. Ships contracted to carry British Royal Mail used the designation RMS. Liners were also the preferred way to move gold and other high value cargos.

The busiest route for liners was on the North Atlantic with ships traveling between Europe and North America. It was on this route that the fastest, largest and most advanced liners travelled. But while in contemporary popular imagination the term "ocean liners" evokes these transatlantic superliners, most ocean liners historically were mid-sized vessels which served as the common carriers of passengers and freight between nations and among mother countries and their colonies and dependencies in the pre-jet age. Such routes included Europe to African and Asian colonies, Europe to South America, and migrant traffic from Europe to North America in the Nineteenth and first two decades of the Twentieth Centuries and to Canada and Australia after the Second World War.