A graph which shows how hydrophobic each amino acid in a polypeptide is versus where it is located on the polypeptide. The hydropathy plot is used to find clusters of hydrophobic amino acids, which could indicate that the polypeptide in question is a transmembrane protein. A transmembrane protein has hydrophilic parts which protrude out on either side of the cellular membrane, and a hydrophobic center which lies within the membrane.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.