A theoretically exact reference from which dimensions and tolerances are measured.
A datum is an ideal plane, axis or point derived from a real feature. It is what makes a GD&T callout measurable: the tolerance zone is defined relative to the datum, not to whatever surface the inspector chooses.
Datum selection is a design decision with real consequences. Choosing a functional surface as the primary datum aligns inspection with how the part actually works; choosing an arbitrary one produces tolerances that pass inspection and fail in service.
Treating the datum feature (the physical surface) and the datum (the ideal reference derived from it) as the same thing. They are not — the feature has imperfections, the datum does not.
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