The Collaborative International Dictionary of English v.0.48:

Benthos \Ben"thos\ (b[e^]n"th[o^]s), n. [NL., fr. Gr. be`nqos
   depth of the sea.]
   The bottom of the sea, esp. of the deep oceans; hence (Bot. &
   Zool.), the fauna and flora of the sea bottom; -- opposed to
   plankton.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

benthos
    n 1: a region including the bottom of the sea and the littoral
         zones [syn: benthos, benthic division, benthonic
         zone]
    2: organisms (plants and animals) that live at or near the
       bottom of a sea