A solution in which the concentration of the solute is lowest at the top and gradually becomes more dense as it gets deeper, and is highest at the bottom. It can be stepwise (each level of density is discrete from other levels of density, and forms a separate layer) or continuous (the density changes gradually).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.