A datum is a theoretically exact reference — a point, axis, or plane — from which the geometric tolerances on a part are established and measured. It is the anchor of the part's coordinate system. When a print calls out the position of a hole, the orientation of a surface, or the runout of a shaft, those controls only have meaning relative to the datums they reference. A datum identified by a letter in a feature control frame (A, B, C, and so on) tells the inspector exactly what to measure from.
It is important to distinguish the datum from the feature it comes from. A real part surface is never perfectly flat and a real hole is never perfectly round, so the datum itself is a perfect geometric ideal — the true plane, axis, or point — while the datum feature is the actual physical surface or hole on the part. In practice the datum is realized by a datum feature simulator, such as a surface plate, a gauge pin, or the jaws of a chuck, that contacts the high points of the real feature and stands in for the ideal reference during inspection.
Datums are usually called out in a sequence that builds a datum reference frame: a primary datum constrains the part first (typically a plane contacting three high points, removing three degrees of freedom), a secondary datum constrains it further against the remaining motion, and a tertiary datum locks the last degree. This ordered, three-2-1 style constraint ensures the part is located the same way every time it is measured, which is what makes repeatable inspection possible.
The practical rule is to choose datums that match how the part is actually located, clamped, or seated in its assembly. If a bracket mounts on a flat face and two bolt holes, those should be the datums, so the tolerance zones reflect how the part really fits and functions. Datums chosen for convenience rather than function can pass parts that do not assemble, or reject parts that work fine. Together with controls like true position and maximum material condition, well-chosen datums are what make GD&T an unambiguous, functional language rather than a set of arbitrary numbers.
A few refinements round out the picture. A datum feature can itself carry a material-condition modifier — for example, a datum hole referenced at maximum material condition lets the simulating gauge pin be sized at the hole's MMC, so the part can shift slightly relative to the datum as the hole grows, which mirrors real bolted assembly and grants bonus clearance. Datum targets (specific points, lines, or small areas, rather than a whole surface) are used when a casting or stamping surface is too irregular to contact reliably along its full extent, pinning the reference to defined high spots instead. And the order in which datums are listed in a feature control frame is not cosmetic: A-B-C and A-C-B can produce different acceptance results because they establish the reference frame in a different sequence. Getting the datum scheme right is therefore the foundation on which every position, orientation, and runout tolerance on the part depends.