Sea lions are sea mammals characterized by external ear flaps, long foreflippers, the ability to walk on all fours, and short, thick hair. Together with the fur seals, they comprise the family Otariidae, eared seals, which contains six extant and one extinct species (the Japanese sea lion) in five genera. Their range extends from the subarctic to tropical waters of the global ocean in both the Northern and Southern Hemispheres, with the notable exception of the northern Atlantic Ocean. They have an average lifespan of 20–30 years. A male California sea lion weighs on average about and is about long, while the female sea lion weighs and is long. The largest sea lion is Steller's sea lion, which can weigh and grow to a length of . Sea lions consume large quantities of food at a time and are known to eat about 5–8% of their body weight (about ) at a single feeding.

In heraldry, the term sea-lion (sometimes called a morse) refers to a legendary creature that has the head and upper body of a lion, but with webbed forelimbs and a fish tail. These occur most frequently as supporters, but also occur as crests and occasionally as charges. Sea-lions are frequently found in "sejant" or "sejant-erect" (sitting upright) attitudes, but may also be found "naiant" (horizontally, as if swimming) or "assurgeant" (issuing from the waves of the sea).