The Lyman-alpha transition (left), together some ofther transitions in the Lyman and Balmer series of the hydrogen atom.
Lyman-alpha is the spectral line at 121.6 nm (1216 Å), in the far ultraviolet, that corresponds to the transition of an electron between the two lowest energy levels of a hydrogen atom. It is the first member of the Lyman series.
The Lyman-alpha forest is the appearance of many differentially redshifted Lyman-alpha absorption lines in the spectrum of a quasar, caused by intervening primordial hydrogen clouds along the line of sight to the quasar. Scientists use the Lyman-alpha forest to trace how matter congregates on the scale of a few megaparsecs.