What is a datum in GD&T and how is it established?

A datum is a theoretically perfect geometric reference (point, axis, or plane) from which measurements are made to locate features on a part. Datums are established from datum features on the actual part, which contact datum simulators (like surface plates or gauge pins) during inspection. The order of datums (primary, secondary, tertiary) in a feature control frame defines how the part is constrained.