Ch 1.5, Ch 5.5 An integer or half-integer labeling the eigenvalues of a set of commuting observables. For the hydrogen atom: $n$ (principal, $n = 1, 2, \ldots$), $l$ (orbital, $0 \leq l \leq n-1$), $m_l$ (magnetic, $-l \leq m_l \leq l$), $m_s$ (spin, $\pm 1/2$). *See also:* Eigenvalue, Good quantum