Natural parity ($\pi =(-1{)}^J$)

For a state of integer total angular momentum $J$, natural parity means $\pi =(-1{)}^J$: $0^{+},1^{-},2^{+},3^{-},\dots$. The other sign is called unnatural parity. The terminology is useful when a spin-zero positive-parity parent emits a spin-zero positive-parity particle: angular momentum conservation fixes relative orbital angular momentum $L=J$, and parity conservation then forces natural parity for the daughter.