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The Lyman limit is the short-wavelength limit of transitions to the hydrogen atom ground state. Its photon energy, approximately $13.6\mathrm{eV}$, is the ground-state ionization energy.

A Lyman limit system has approximately unit or greater optical depth at the neutral-hydrogen Lyman limit near $91.2\mathrm{nm}$. The threshold follows from ${\tau }_{912}=N_{\mathrm{HI}}{\sigma }_{912}$ with ${\sigma }_{912}\simeq 6.3\times 10^{-18}{\mathrm{cm}}^2$. Its strong continuum absorption distinguishes it from ordinary weak Lyman-alpha forest lines. Classification conventions often separate damped Lyman-alpha systems as the highest-column category.