The collection of marine and freshwater organisms that can swim freely, independent of water movement.
Examples of nekton include lobsters, squid, and most fish.
In contrast, 'plankton' are floaters and drifters, generally unable to swim freely. Jellyfish begin their lives as plankton and become nekton when they are big enough to swim.
Etymology: 1890s, from Greek nektos, swimming.
I suppose the boxfish is considered nekton, but it seems a near thing: their bodies are rigid, and they move by ostraciiform swimming (basically, wagging the tail); the result is extremely slow forward motion, and a tendency to get sucked into aquarium filters.