Any material (concrete blocks, reeds, wood, dirt,
gravel, etc.) used for the primary purpose of replacing an
aquatic area with dry land or of changing the bottom
elevation of a waterbody. The term does not include any
pollutant discharged into the water primarily to dispose of
waste.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.