datum

Something known or assumed as fact, and is made the basis of reasoning or inference which an intellectual system of any sort (such as knowledge or theoretical framework) is constructed.

A point, line, plane, or surface with reference to which positions (such as elevations) are measured or indicated. (Examples include a permanent benchmark in leveling or mean sea level in a topographical survey).

A floating reference point, or SLDMB, used to evaluate surface currents in a body of water, and often employed by coastal search and rescue.