midpoint, centre, center - a point equidistant from the ends of a line or the extremities of a figure

center of buoyancy, center of immersion, centre of buoyancy, centre of immersion - (physics) the center of mass of the immersed part of ship or other floating object

barycenter - (astronomy) the common center of mass around which two or more bodies revolve

centroid - the center of mass of an object of uniform density