Plankton are a diverse collection of organisms found in water or air that are unable to propel themselves against a current or wind. They are carried by tides and currents and cannot swim well enough to move against these forces. Plankton are usually microscopic, often less than one inch in length, but they also include larger species like some crustaceans and jellyfish. The name plankton was coined by German marine biologist Victor Hensen in 1887 from shortening the word halyplankton from Greek ᾰ̔́λς háls "sea" and πλανάω planáō to "drift" or "wander".

Plankton can be classified in several ways, including by size, type, and how long they spend drifting. But the most basic categories divide plankton into two groups: phytoplankton (plants) and zooplankton (animals) . Phytoplankton are microscopic plants that perform photosynthesis to convert the sun’s rays into energy to support them, and they take in carbon dioxide and produce oxygen. They are found near the water’s surface because they need the sun’s energy. Zooplankton include microscopic animals, the young of larger invertebrates and fish, and weak swimmers like jellyfish. Most zooplankton eat phytoplankton, and most are, in turn, eaten by larger animals (or by each other) .

Plankton are an important food source for many large and small ocean creatures. They also play a vital role in absorbing carbon dioxide and producing oxygen in the ocean. The amount and distribution of plankton depends on available nutrients, the state of water, and a large amount of other plankton. The study of plankton is termed planktology and a planktonic individual is referred to as a plankter. The adjective planktonic is widely used in both the scientific and popular literature and is a generally accepted term.