/ˈplaNGktən/
1. Marine and freshwater organisms that, because they are nonmotile or because they are too small or too weak to swim against the current, exist in a drifting, floating state. The term plankton is a collective name for all such organisms and includes certain algae, bacteria, protozoans, crustaceans, mollusks, and coelenterates, as well as representatives from almost every other phylum of animals. Plankton is distinguished from nekton, which is composed of strong-swimming animals, and from benthos, which includes sessile, creeping, and burrowing organisms on the seafloor. (global.britannica.com)
2. The small and microscopic organisms drifting or floating in the sea or fresh water, consisting chiefly of diatoms, protozoans, small crustaceans, and the eggs and larval stages of larger animals. Many animals are adapted to feed on plankton, esp. by filtering the water. (Google Dictionary)
3. The passively floating or weakly swimming usually minute animal and plant life of a body of water. (merriam-webster.com)
4. Any organisms that live in the water column and are incapable of swimming against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales. (wikipedia.org)
5. A generic term for all the organisms that float in the sea. A single organism is known as a plankter. (wiktionary.org)
Word origin: From Greek plankton, neuter of planktos “wandering, drifting,” verbal adjective from plazesthai “to wander, drift,” from plazein “to drive astray.”