A sea lane or shipping lane is a regularly used route for ocean-going and Great Lakes vessels. In the time of sailing ships they were not only determined by the distribution of land masses but also the prevailing winds, whose discovery was crucial for the success of long voyages. Sea lanes are very important for trade by sea.

The establishment of the North Atlantic sea lanes was inspired by the sinking of the US mail steamer SS Arctic by collision with the French steamer SS Vesta in October 1854 which resulted in the loss of over 300 lives. Lieutenant M. F. Maury of the US Navy first published a section titled "Steam Lanes Across the Atlantic" in his 1855 Sailing Directions proposing sea lanes along the 42 degree latitude. A number of international conferences and committees were held in 1866, 1872, 1887, 1889, and 1891 all of which left the designation of sea lanes to the principal trans-Atlantic steamship companies at the time; Cunard, White Star, Inman, National Line, and Guion Lines. In 1913-1914 the International Convention for Safety of Life at Sea held in London again reaffirmed that the selection of routes across the Atlantic in both directions is left to the responsibility of the steamship companies.

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean. It is also used sometimes to describe a large saline lake that lacks a natural outlet, such as the Caspian Sea.

Arctic (belonging to the Arctic Ocean) and Antarctic (Southern Ocean) seas, as well as some other seas freeze in winter. This occurs below the freezing point of pure water, at about -1.8 °C (28.8 °F). Frozen salt water becomes sea ice.

Humans navigated seas from antiquity. Ancient Egyptians and Phoenicians navigated the Mediterranean Sea and the Red Sea. Hannu was the first see explorer of whom there is any information. He sailed along the Red Sea and further to the Arabian Peninsula and the African Coast around 2750 BC. In the 1st millennium BC, Phoenicians and Greeks established colonies all over the Mediterranean, including its outlets like the Black Sea. The seas along the eastern and the southern Asian coast were used by Arabs and Chinese for navigation, and the North Sea and the Baltic Sea were known to Europeans in Roman times. Other seas were not used for navigation in the antiquity and were actually discovered.

A lane is a part of the roadway (British: carriageway) within a road marked out for use by a single line of vehicles in such a way as to control and guide drivers for the purpose of reducing traffic conflicts. Most public roads (highways) have at least two lanes, one for traffic in each direction, separated by Lane markings. Major highways often have two roadways separated by a median, each with multiple lanes. A single-track road carries traffic in both directions within a single lane with passing places to allow vehicles to pass. In North America and Australia, the term also may refer to rear access roads which act as a secondary vehicular network in cities and towns. A minor rural road may be referred to as a country lane; some urban streets which began as country lanes are still called lanes, such as Drury Lane in London.

The U.S. Interstate Highway System uses a 12-foot (3.7 m) standard for lane width. 11-foot (3.4 m) lanes are found to be acceptable by the Federal Highway Administration for automobile traffic, but as lane width decreases (9-foot (2.7 m) lanes are found in some areas) traffic capacity decreases. A full-width freeway lane typically has a capacity of 2,000 cars per hour.