relative molecular mass, M r

https://dx.doi.org/10.1351/goldbook.R05271
Ratio of the mass of a molecule to the unified atomic mass unit. Sometimes called the molecular weight or relative molar mass.
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 990