The Collaborative International Dictionary of English v.0.48:

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.]

WordNet (r) 3.0 (2006):

plankton
    n 1: the aggregate of small plant and animal organisms that
         float or drift in great numbers in fresh or salt water