1. n. [Geology]

Minute organisms that float or drift passively near the surface of oceans and seas. Plant-like plankton, or phytoplankton, include diatoms. Zooplankton are animals that have a limited ability to move themselves. The changes in plankton over time are useful for estimation of relative ages of rocks that contain the fossilized remains of plankton.

See: diatom,  relative age,  stratigraphy