Baryon number

The baryon number $B$ is a quantum number representing baryons, which are hardons with three quarks. It is equal to 1 for baryons and -1 for antibaryons. More technically, it is dependent on the number of quarks:

where $n_\text{quark}$ is the number of quarks in the hadron and $n_\text{antiquark}$ is the number of antiquarks. Since all known baryons either have three quarks or three antiquarks, this comes out to always be 1 or -1. Mesons always have one quark and one antiquark, so $B=0$ for all of them, as we should expect.

In the Standard Model, the baryon number is always conserved regardless of fundamental interaction. Many theories beyond the Standard Model find this to not always be true in more exotic scenarios.