The exact value a measurand would show if a perfect measurement were possible. Because every measurement carries uncertainty, the true value is never known; labs work instead with best estimates and with reference values assigned by higher-level standards, sometimes called conventional values. Uncertainty-based practice describes the doubt about the result rather than claiming a distance from an unknowable truth.
Nobody in this building knows the true value, we know what the reference says and how sure we are of it.
Veterans avoid writing 'true value' in reports; 'reference value' or 'best estimate' says what you actually have.
Related terms
Error
The signed difference between a measured value and the reference value of the quantity being measured.
Uncertainty
A number that states how much doubt surrounds a measured value, as a standard deviation or as a stated interval.
Measurand
The exact quantity a measurement is aimed at, pinned down tightly enough that everyone measures the same thing.
Bias
The estimated systematic offset of a measuring system: its average reading minus the reference value.