Nekton is the counterpart to plankton. Whereas plankton are all those small organisms that drift freely in the oceanic currents, nekton are those that do not drift, but swim. As a rule, they tend to be larger creatures, and include everything from sardines to whales.
The term was coined in 1893 by biologist Ernst Haeckel, who was looking for a counterpart to the recently coined 'plankton' (Viktor Hensen, 1891). He took the Greek nekton, the neuter of nektos, 'swimming'), exactly mirroring the use of Greek planktos meaning 'wandering' or 'drifting'.
The term is falling out of use today, as there is no firm dividing line between the nekton and plankton. Many organisms switch between the two modes depending on life stage, and many plankton do have significant motility, even if it is not easily apparent to gigantic human observers.
Large animals do in fact move about more, as the viscosity and movement of water has a proportionally smaller effect on them. However, it is also the case that many very small animals paddle themselves about; we just tend to ignore the few inches they move, simply because they work on a different scale than humans do. Jellyfish are a common example of this; small jellyfish do swim, but are often considered plankton; once they become large enough to be easily seen by the naked eye, they are considered nekton. Indeed, even very large examples of nekton, such as the sunfish, start life out as small, motile plankton.