An Etymological Dictionary of Astronomy and Astrophysics

The continuum trough observed in the spectra of high redshift quasars (z> 6) at the blue wing of their Lyman-alpha emission line (1216 Å). It is explained by the
scattering of the radiation of the quasar by intergalactic neutral hydrogen on the line of sight. Because of the cosmological expansion, the quasar line is redshifted
with respect to the continuum trough. The Gunn-Peterson opacity increases rapidly with redshift. It is interpreted as a strong evidence for the reionization of the Universe around z = 6.

See also: After James E. Gunn and Bruce A. Peterson who predicted the effect in 1965; → effect.