Source : Webster's Revised Unabridged Dictionary (1913)

Supernatant \Su`per*na"tant\, a. [L. supernatanus, p. pr. of
   supernatare to swim above; super above + natare to swim.]
   Swimming above; floating on the surface; as, oil supernatant
   on water.

Source : WordNet®

supernatant
     adj : of a liquid; floating on the surface above a sediment or
           precipitate; "the supernatant fat was skimmed off"
     n : a liquid lying above a sediment (floating on the surface)