= Spin gap
A <spin gap> is a nonzero minimum <energy> required to reach a specified magnetic excitation above the <ground state>. One must specify which <spin> sector or measurement couples to that excitation; not every excitation gap is a <spin gap>.
Two antiferromagnetically coupled spin-$1/2$ particles illustrate it. For $H=J\mathbf S_1\cdot\mathbf S_2$ with dimensionless <spin> operators and $J>0$, the singlet has <energy> $-3J/4$ and the triplet has $J/4$, giving gap $J$. Thermal magnetic response is suppressed when $k_BT$ is well below that scale. Extended systems require their collective excitation spectrum rather than this isolated-pair argument alone.
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