Nautilus (from Greek ναυτίλος 'sailor') is the common name of any marine creatures of the cephalopod family Nautilidae, the sole family of the suborder Nautilina. It comprises six species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to the species Nautilus pompilius, the name chambered nautilus is also used for any species of the Nautilidae.

Having survived relatively unchanged for millions of years, nautiluses represent the only living members of the subclass Nautiloidea, and are often considered to be "living fossils."

The nautilus shell presents one of the finest natural examples of a logarithmic spiral (it is sometimes incorrectly claimed to be a golden spiral as well).

The name "Nautilus" originally referred to the Argonauta, otherwise known as paper nautiluses, because they allegedly use their two expanded arms as sails (cf. Aristotle Historia Animalium 622b).

First Nuclear Submarine USS Nautilus (SSN-571) 1954
On 12 December 1951, the Navy Department announced that the world's first nuclear-powered submarine, SSN-571, would carry the name Nautilus. Authorized by the Congress in July 1951, her keel was laid at the Electric Boat Division of General Dynamics in Groton, Connecticut by Harry S. Truman, President of the United States, on 14 June 1952. A year and a half later—on 21 January 1954—the First Lady, Mrs. Dwight Eisenhower broke the traditional bottle of champagne on her bow as the ship slid down the ways into the Thames River in Connecticut. "Underway on nuclear power" On 17 January 1955, USS Nautilus put to sea for the first time and signaled her historic message "Underway on nuclear power." She steamed submerged 1,300 miles from New London to San Juan, Puerto Rico, in just 84 hours. The success of Nautilus ensured the future of nuclear power in the Navy.