In nature, plankton (singular plankter) are creatures that live in a water column and can't swim against a significant large scale water current. They provide a crucial source of food to many large aquatic (fresh water or ocean) creatures such as fish and whales.
Plankton often include a wide range of animals that vary from microscopic to relatively large(over tens of centimetres). Many are crustaceans or jellyfish.
Origin of the name plankton
The name plankton is derived from the Greek adjective πλαγκτός - planktos, meaning errant, and by extension "wanderer" or drifter. It was coined by Victor Hensen. Plankton typically flow with ocean currents.