GUM = Guide to the Expression of Uncertainty in Measurement
The GUM is codified in several related standards, several of which are national standards that republish the GUM.
The GUM is developed and published by the International Bureau of Weights and Measures (BIPM at www.bipm.org)
Link to the GUM online (http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf)