cruise ship or cruise liner is a passenger ship used for pleasure voyages, where the voyage itself and the ships amenities are part of the experience, as well as the different destinations along the way. Transportation is not the prime purpose, as cruise ships operate mostly on routes that return passengers to their originating port, so the ports of call are usually in specified region of a continent.

In contrast, dedicated transport oriented ocean liners do "line voyages" and typically transport passengers from one point to another, rather than on round trips. Traditionally, an ocean liner for the transoceanic trade will be built to a higher standard than a typical cruise ship, including high freeboard (Freeboard (nautical)) and stronger plating to withstand rough seas and adverse conditions encountered in the open ocean, such as the North Atlantic. Ocean liners also usually have larger capacities for fuel, victuals, and other stores for consumption on long voyages, compared to dedicated cruise ships.

However, the lines between both types of passenger ships have blurred, particularly with respect to deployment. Larger cruise ships have also engaged in longer trips such as transocean voyages which may not lead back to the same port for many months (longer round trips). Some former ocean liners currently operate as cruise ships, such as MS Marco Polo (MS Marco Polo) and MS Mona Lisa (MS Mona Lisa), however this number is ever decreasing. The only dedicated transatlantic ocean liner in operation as a liner, as of February 2010, is the Queen Mary 2 of the Cunard (Cunard Line) fleet, and she also sees significant service on cruise routes.

Cruising has become a major part of the tourism industry, accounting for U.S.$27 billion with over 18 million passengers carried worldwide in 2010. The worlds largest cruise liner is Royal Carribean Internationals Oasis of the Seas. The industrys rapid growth has seen nine or more newly built ships catering to a North American clientele added every year since 2001, as well as others servicing European clientele. Smaller markets such as the Asia-Pacific region are generally serviced by older tonnage displaced by new ships introduced into the high growth areas.