Charm (symbol C) is a flavour quantum number representing the difference between the number of charm quarks (c) and charm antiquarks (c) that are present in a particle:[1][2] By convention, the sign of flavour quantum numbers agree with the sign of the electric charge carried by the quarks of corresponding flavour.
The charm antiquarks have the opposite charge (Q = −2⁄3), and flavour quantum numbers (C = −1).
[3] As with any flavour-related quantum numbers, charm is preserved under strong and electromagnetic interaction, but not under weak interaction (see CKM matrix).
For first-order weak decays, that is processes involving only one quark decay, charm can only vary by 1 (ΔC= ±1,0).
Since first-order processes are more common than second-order processes (involving two quark decays), this can be used as an approximate "selection rule" for weak decays.