Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

plankton
     n : the aggregate of small plant and animal organisms that float
         or drift in great numbers in fresh or salt water