A number added to a raw result to cancel a known systematic error; it has the same size as the estimated error and the opposite sign. A thermometer known to read 0.15 °C high gets a correction of -0.15 °C, and a gauge block measured at 21 °C gets a thermal expansion correction back to 20 °C. Applying a correction does not remove its uncertainty; the uncertainty of the correction itself stays in the budget.
Apply the cert correction of minus 2.1 ppm to the 10 volt before you compare it to the UUT.
Watch the sign: the correction is the negative of the error, and a flipped sign doubles the problem instead of removing it.
Related terms
Systematic error
Error component that pushes every repeat reading the same way, or follows a pattern you can predict and correct.
Bias
The estimated systematic offset of a measuring system: its average reading minus the reference value.
Error
The signed difference between a measured value and the reference value of the quantity being measured.
Influence quantity
A condition that is not the thing being measured but still changes the reading, such as temperature or humidity.