Definition

History
In 1818, Black Ball Line, with a fleet of clipper ships, offered the first regular passenger service with emphasis on passenger comfort, from England to United States. From the early 1800s, steam engines began to appear in ships, but initially they were inefficient and offered little advantage over sailing ships.
The clipper domination was challenged when SS Great Western, designed by railway engineer Isambard Kingdom Brunel, began its first Atlantic service in 1837. It took 15 days to cross the Atlantic compared to two months for sail powered ships. Unlike the clippers, the steamers offer a consistent speed and the ability to keep up schedules. The early steam ships still had sails as well though, as engines at this time still required large quantities of fuel. Having sails enabled vessels like the Great Western to take advantage of favourable weather conditions and minimise fuel consumption.
In 1840, Cunard Line's Britannia began its first regular passenger and cargo service by a steamship, sailing from Liverpool to Boston. Despite some advantages offered by the steamships, clippers remained dominant. In 1847, SS Great Britain became the first screw driven ship with an iron hull to cross the Atlantic. More efficient propellers began to replace bulkier paddle wheels, found on earlier ocean liners.
In 1870, White Star Line's RMS Oceanic set a new standard for ocean travel by placing the first class cabin amidships, adding large portholes and offering running water and electricity. The size of ocean liners increased from 1880 onward because of massive immigration to United States.
RMS Umbria and her sister ship RMS Etruria were the last two Liners that were fitted with auxiliary sails. Umbria was built by John Elder & Co of Glasgow, Scotland in 1884. Umbria and Etruria by the standards of the time were record breakers. They were the largest liners then in service and they plied the Liverpool to New York Service.

The 19th century
The period between the end of the 19th century and World War II is considered the "golden age" of ocean liners. Driven by strong demand created by European emigration to the United States and Canada, international competition between passenger lines and a new emphasis on comfort, shipping companies built ever larger and faster ships.
Canadian Pacific Railway became one of the largest transportation system in the world combining with ships and railways operating from Canada. In 1891 CPR shipping division began its first Pacific operation. In 1903, CPR began its first Atlantic service because of rising migration of Europeans to western Canada as the result of free land offered by the Canadian government.
Since the 1830s ships had unofficially been competing for the honor of making the fastest Atlantic crossing. This honor came to be known as the Blue Riband; in 1897 Germany took the award with a series of new ocean liners, starting with the SS Kaiser Wilhelm der Große. In 1905, British company Cunard fitted their liner Carmania with steam turbines and it outperformed its near identical sister Caronia, which was powered by triple expansion steam engines. At the time, these were the largest ships in the Cunard fleet, and the use of the different propulsion methods in otherwise similar ships allowed the company to evaluate the merits of both. The engines in Carmania proved to be a success and consequently in 1907, the Cunard Line introduced the much larger Lusitania and Mauretania, both powered by steam turbines. The Mauretania would go on to hold the Blue Riband for an astonishing 20 years.
Cunard's dominance of the Blue Riband did not keep other lines from competing in terms of size and luxury. In 1910 White Star launched the Olympic, the first of a trio of 45,000 plus gross ton liners that was also to include the Titanic and Britannic. These ships were almost 15,000 tonnes larger and 100 feet (30 m) longer than the Lusitania and Mauretania.
Hamburg-America Line also ordered three giant ships, Imperator, Vaterland and Bismarck; all over 51,500 gross tons. The first of these ships, Imperator, was launched in 1912. The largest, Bismarck, would be the largest ship in the world until 1935. These ships would see little or no service with Hamburg-America before World War I. After the war they were seized as war reparations and given to British and American lines.
The surge in ocean liner's size outpaced the shipping regulations. In 1912, Titanic sank after hitting an iceberg, claiming more than 1,500 lives. A factor contributing to the high loss of life was that there were not enough lifeboats for everyone. After the Titanic disaster, the regulation was revised to require all ocean liners to carry enough lifeboats for all passengers and crew. In addition, International Ice Patrol was established to monitor the busy north Atlantic shipping lanes for icebergs.
Until the 1920's most shipping lines relied heavily on emigration for sales and they were hard hit when the US Congress introduced a bill to limit immigration into the United States. As the result, many ships took on cruising and the least expensive cabins were reconfigured from third class to tourist class. To make matters worse, the great depression put many shipping lines into bankruptcy.
Despite the harsh economic conditions, a number of companies continued to build larger and faster ships. In 1929 the German ships Bremen and Europa beat the crossing record set by the Mauretania 20 years earlier with an average speed of almost 28 knots (52 km/h). The ships used bulbous bows and oil fired boilers to reach these high speeds while maintaining economical operating costs. In 1933 the Italian 51,100 ton ocean liner SS Rex, with a time of four days and thirteen hours, captured the westbound Blue Riband, which she held for two years. In 1935 the French liner SS Normandie used a revolutionary new hull design and powerful turbo-electric propulsion to take the Blue Riband from the Rex. Because of the poor economic conditions the British government forced Cunard Line and White Star Line to merge. The newly merged company countered with its liners RMS Queen Mary and RMS Queen Elizabeth; the Queen Mary was to hold the Blue Riband from 1936-37 and 1938-52.
The post World War II era was a brief but busy period. Notable transatlantic liners included the SS United States, which was the last ocean liner to hold the Blue Riband and the SS France, which held the record for the longest passenger ship from when she entered service in 1961 until the launch of RMS Queen Mary 2 in 2003. Australian Government sponsored immigration resulted in a busy trade between Europe and Australia, producing such notable ships as the SS Oriana and SS Canberra. These two, operating on the P&O-Orient Line service, were the last, largest and fastest liners built for the Australian route.

The 20th century
Prior to World War II, aircraft weren't a huge threat to ocean liners. Most pre-war aircraft were noisy, cramped and vulnerable to bad weather, few had the range needed for transoceanic flights, and all were expensive and had a small passenger capacity. However, World War II accelerated the development of aircraft. Four engine bombers such as the Avro Lancaster and Boeing B-29, with their long range and massive carrying capacity, were a natural prototype for a next generation airliner. Jet aircraft technology also accelerated after the development of jet aircraft for military use in World War II. In 1953, the De Havilland Comet became the first commercial jet airliner; the Sud Aviation Caravelle, Boeing 707 and Douglas DC-8 followed later. The T/S Michelangelo and T/S Raffaello, built in 1962 and 1963 for Italian Line, were two of the last ocean liners to be built primarily for liner service across the North Atlantic, as in the 1960s airlines gradually took over the business formerly done by ships. By the early 1970s passenger ships were used almost exclusively for cruising.
After the end of the large scale passenger liner business, many ships continued in use as cruise ships; as of 2003 a small number of former liners were still in service. A few more, such as RMS Queen Mary, are still afloat but permanently docked and used for other purposes; in the case of the Queen Mary, as a museum ship. The only large liner still used on scheduled line voyages in 2006 is Cunard Line's RMS Queen Mary 2, which replaced the line's Queen Elizabeth 2 on the transatlantic route in 2004. QE2 made her maiden voyage in 1969 and was the only major operational ocean liner for several decades, but now is given over to cruising.

The end
Ocean liners played a major role in World War I. Large ocean liners, such as Mauretania and Olympic, were used as troopships and hospital ships while smaller ocean liners were converted to armed merchant cruisers. The Britannic, sister to the Titanic and Olympic, never served on the liner trade for which she was built, instead entering war service as a hospital ship as soon as she was completed; she lasted a year before being sunk by a mine. Some other liners were converted to Q-ships purposely designed to hunt down submarines. In 1915, Lusitania, still in service as a civilian passenger vessel, was torpedoed by a German U-boat with many causalties.
Ocean liners such as Queen Mary and Queen Elizabeth were used again in World War II, although not as merchant cruisers--they were mostly used as troopships and hospital ships. Normandie sailed to the United States in 1942 for conversion to a troopship, but never saw service in that role. A fire during the conversion work caused her to capsize because of the water poured into her full while docked in New York. Salvage attempts failed and she was scrapped in 1946. The majority of the superliners of the '20s and '30s were victims of U-boats, mines or enemy aircraft. The Empress of Britain was attacked by Nazi planes, then torpedoed by a U-boat when tugs tried to tow her to safety. She was the largest British ocean liner to sink during World War II. Germany's speed queen Bremen fell victim to a disgruntled crew member in 1941 - she was set aflame and became a total loss. Italy's giants, the Rex and the Conte di Savoia were destroyed by the British RAF and the retreating Nazis, respectively. The United States lost the American President Lines' SS President Coolidge to, of all things, an Allied mine in the South Pacific. No shipping line was untouched by World War II.
More recently, during the Falklands War, three ships that were either active or former liners were requisitioned for war service by the British Government. The Cunard liner QE2 and P&O cruise ship and former England to Australia liner Canberra served as troopships, carrying British Army personnel to Ascension Island and the Falkland Islands to recover the Falklands from the invading Argentine forces. The P&O educational cruise ship and former British India Steam Navigation Company liner Uganda was requisitioned as a hospital ship and, after the war, served as a troopship until an airport was built at Port Stanley that could handle RAF trooping flights.

Famous and infamous

List of ocean liners
Transatlantic
Packet ship