Abstract
The 2019 redefinition of the International System of Units (SI) is often described as a historic transition from unit definitions based on artefacts or physical phenomena to a system underpinned by fundamental constants of nature. The current SI defines the SI base units, and from them all SI units, in terms of defining constants: these provide the basis for stable and comparable measurement. Accounts of the recent history of the SI often discuss the transition to the use of defining constants but rarely seek to investigate the concept ‘defining constant’ as such. This review article revisits the evolution of the role of defining constants in the SI, by examining its philosophical and practical implications in metrology for the purposes of unit definition and investigating the different ways in which SI units can be said to depend on defining constants. It also highlights the ongoing discussions surrounding the physical significance and interpretational nuances of defining constants within the framework of the SI and the choices that were made during the 2019 revision of the SI.