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, currents, and other forces, which determine where they go. 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 are classified 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. 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 the foundation of freshwater and seawater food pyramids. They play a crucial role in the marine food chain and are an important food source for many large and small ocean creatures. Without plankton, there would be few living organisms on earth, and certainly no Great Barrier Reef. Planktonic organisms are also threatened by microplastics, which can block phytoplankton from receiving enough sunlight to survive.