The Greek πλανκτόν is "wandering."

Plankton is a set of organisms inhabiting the water column of continental and marine water bodies, unable to resist transport by currents, passively transported by water movement, and capable of active vertical migration. Plankton includes phyto- (microscopic algae), zoo- (rotifers, worms, paddlefish and branchiopod crustaceans, jellyfish, eggs, larvae, fish fry and many small animals) and bacterioplankton. In fresh waters, a distinction is made between lake plankton (limnoplankton) and river plankton (potamoplankton). Plankton includes not only small fish, inactive inhabitants of the deep layers of the pelagial, but also fish that move with the help of undulatory movements of fins, unable to resist the oncoming movement of water and passively carried by the current sometimes for considerable distances, despite their impressive size (anglers, moonfish, herring kings).