(time when the universe's ionized hydrogen atoms neutralized)
In cosmology, recombination is a point in the history of the
universe around redshift 1090, at about 378k years after
the Big Bang, when electrons and protons
paired up to form neutral hydrogen atoms.
In this usage, the term is a misnomer because
this "recombination" is actually the first time
the electrons and protons were together.
This was the end of the photon epoch: when photons'
mean free path (between electrons and protons) was
short due to constant Compton scattering by electrons,
which had made space opaque. This change
is also termed the photon decoupling,
removing one of the conditions maintaining the previous
thermodynamic equilibrium,
the previous balance of interactions between matter and photons that
maintained some particular energy-distribution of each.
Recombination was the beginning of the dark age:
the universe was transparent but there were not yet stars.
The freed photons traveled freely,
many having no subsequent interaction with matter
(thus they were free streaming),
and now constitute the cosmic microwave background (CMB).
The term recombination has a more general meaning: the joining
of positive ions and electrons into neutral atoms.
This also occurs in other common situations, including many
astronomical circumstances, such as the interior of stars.
Emission resulting from such recombination
is termed free-bound emission.