reference state

of an element
https://doi.org/10.1351/goldbook.R05233
The state in which the element is @[email protected] at a chosen standard-state pressure and for a given temperature.
Source:
PAC, 1982, 54, 1239. (Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions) on page 1249 [Terms] [Paper]