https://doi.org/10.1351/goldbook.08169
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.