The share of uncertainty that comes from an incompletely specified measurand, which no instrument upgrade can remove. Asking for 'the diameter' of a shaft that is slightly oval and tapered leaves a spread of possible values no matter how good the instrument is. Drawings that call out location, method and temperature shrink it, which is why a careful measurand definition pays off.
The part is 4 microns out of round, so the definitional uncertainty on diameter swamps anything the CMM contributes.
Related terms
Measurand
The exact quantity a measurement is aimed at, pinned down tightly enough that everyone measures the same thing.
Uncertainty
A number that states how much doubt surrounds a measured value, as a standard deviation or as a stated interval.
Uncertainty budget
A table listing every uncertainty source in a measurement with its size, distribution, divisor and contribution to the total.