From WordNet (r) 2.0 (August 2003) [wn]:
plankton
n : the aggregate of small plant and animal organisms that float
or drift in great numbers in fresh or salt water
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Plankton \Plank"ton\ (pl[a^][ng]k"t[o^]n), n. [NL., fr. Gr.
plagto`n, neut. of plagto`s wandering, pla`zesqai to wander.]
(Biol.)
All the animals and plants, taken collectively, which live at
or near the surface of salt or fresh waters. --
Plank*ton"ic, a.
[Webster 1913 Suppl.]