Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water (usually oceans or lakes) able to move independently of water currents. Nekton are contrasted with 'plankton' which refers to the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily comprising tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute predators.

Oceanic nekton comprises animals largely from three clades

• Vertebrates form the largest contribution, these animals are supported by either bones or cartilage.
• Molluscs are animals like octopodes, squids, and scallops.
• Crustaceans are animals like shrimp and crabs.

The term nekton was coined in 1890 by Ernst Haeckel; it is rooted in the Greek word nēktón ("the swimming"). The study of swimming organisms (biofluidynamics, biomechanics, functional morphology of fluid locomotion, locomotor physiology) is called nektology. One who studies swimming in all its forms is called a nektologist.

• plankton (the organisms that float or drift within the water)
• neuston (the organisms that float on the water)
• benthos (the organisms at the bottom of a body of water)