Quantity (omitted if zero) attached to each atom in a Lewis structure according to
\[Z_{\rm{formal}} = N_{\rm{valence}} - N_{\rm{lonepairs}} - ½N_{\rm{bonds}}\] Examples: \(\ce{CH2=N^{+}=N^{-}}\), \(\ce{H3O^{+}}\), \(\ce{(CH3)2C=N-O^{-}}\), etc.
Note: This formalism assumes that electrons in bonds are shared equally, regardless of electronegativity.
Source:
PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 421 (https://doi.org/10.1515/pac-2018-1010)