a piece of land on which a flour mill was or could be built; specifically, a lot granted to a person who undertook to build and operate a flour mill there.
1796  (1932)  Read the Petition of John Laurence Esquire, praying for a Mill Seat on the River Humber.
1816  (1818)  The number of abandoned mill-seats, particularly in parts of the country recently settled, as well as the difficulty of working many of those still in use, shew the same process of draining to be still continuing.
1829  These engines he obtains by procuring for himself . . . a mill-seat, or what the Yankees call a hydraulic privilege, which he enjoys by setting himself down by the side of a rapid of some river or other, as there he may erect as many mills as he pleases.
1833  (1963)  The fort is to be erected along the bank of a streamlet, which in its devious course through plain presents points well adapted for Mill-seats. . . .