standard concentration

https://doi.org/10.1351/goldbook.S05909
Chosen value of amount concentration denoted by c⦵ or c° and usually equal to 1 mol dm-3.
See also: standard molality
Source:
Green Book, 2nd ed., p. 54 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)