Symbols characterizing the states of atoms and molecules in terms of multiplicity, symmetry of the total electronic wavefunction, and sometimes total (orbital + spin) angular momentum. Examples: 2P1/2 denotes an atomic state of multiplicity 2 (doublet), electron orbital angular momentum quantum number L = 1, and orbital plus spin angular momentum quantum number J = 1/2.
Source:
Green Book, 2nd ed., p. 28 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)