Plankton (noun)

1. Microscopic plants and animals that float or drift in great numbers in fresh and salt water, forming the basis of the food chain.
2. Microscopic aquatic organisms that drift with the currents in oceans, seas and freshwater bodies.

Origin:

Late 19th century: from greek planktos "wandering" + -on.

Examples:

1. Plankton is a vital part of the ocean's ecosystem.
2. The whale shark is a filter feeder, it feeds on plankton.
3. The research team is studying the effects of pollution on plankton populations.
4. Dinoflagellates, one of the groups of plankton, are important primary producers in the ocean.
5. The study of plankton is called planktology.