Measurement Uncertainty Redefined

On July 2nd, 2025, the Joint Committee for Guides in Metrology (JCGM) organized a widely attended webinar (https://www.youtube.com/watch?v=iHivi6UC5Uk) to present and discuss a new definition of measurement uncertainty being considered as a potential replacement for the current definition in the International Vocabulary of Metrology. The original version of the new definition first appeared in Possolo (2015) and was adopted by the National Institute of Standards and Technology (NIST) of the United States: ``Measurement uncertainty is the doubt about the true value of the measurand that remains after making a measurement. Measurement uncertainty is described fully and quantitatively by a probability distribution on the set of values of the measurand.'' This definition was inspired by an earlier, similar characterization of measurement uncertainty that Stephanie Bell (National Physical Laboratory, United Kingdom) published in 1999 (Bell, 1999). The present short communication first compares definitions of measurement uncertainty and reviews the ``error'' and ``uncertainty'' approaches featured in the introduction to the 3rd edition of the VIM. Next it explains the principal motivation for the redefinition, which was a critical review of the current definition by Thompson (2011). The third section discusses the relation between doubt, incomplete knowledge, and uncertainty, and the concern that words like ``doubt'' and ``uncertainty'' can cast a negative light upon the science and technology that produce measurement results, in particular in laboratory medicine. A closing Coda highlights the principal benefits of the proposed redefinition.