The sea otter (Enhydra lutris) is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg (30 and 100 lb), making them the heaviest members of the weasel family, but among the smallest marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. Although it can walk on land, the sea otter lives mostly in the ocean.

The sea otter inhabits offshore environments, where it dives to the sea floor to forage. It preys mostly on marine invertebrates such as sea urchins, various molluscs and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. First, its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems. Its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.

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.

An otter is any of 13 living species of semiaquatic (or in one case aquatic) mammals which feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals.

The otter subfamily Lutrinae forms part of the family Mustelidae, which also includes weasels, polecats, badgers, and wolverines.

The word otter derives from the Old English word otor or oter. This and cognate words in other Indo-European languages ultimately stem from the PIE root *wódr̥ which also gave rise to the English word "water".

An otter's den is called a holt or couch. A male otter is a dog, a female a bitch, and a baby a whelp, kit, or pup.^{[citation needed]} The collective nouns for otters are bevy, family, lodge or romp, being descriptive of their often playful nature, or when in water, raft.

The time of gestation in otters is about 60 to 86 days. The newborn pup is taken care of by the mother, the father, and all the other offspring. Female otters reach sexual maturity at approximately two years of age, while males can produce offspring at approximately three years of age. After one month, the young otter can come out of the cave, and after two months, it is able to swim. It lives with its family for about one year, so it can learn and be kept safe until maturity. Otters live up to 16 years.