The epoch of reionization (EOR or EoR or just reionization or the era of reionization) is a period in the history of the universe that occurred over time, sometime within about 6 < z < 20 (or 6-15: cited boundaries vary) when the hydrogen atoms that were spread throughout space lost their electrons. Though this is a lengthy period rather than an instant, for some purposes it can be treated as a point in time.
Before the EOR was a period termed the dark age, and just before that, at z = 1089, 378k years after the Big Bang, hydrogen went from ionized to neutral, i.e., recombination. Prior to recombination, the free protons and electrons scattered photons, and the universe was a glowing fog, and recombination, the capture of the electrons made the universe basically transparent, though still without stars (thus the name dark age). When stars and galaxies formed, initially the gas between them remained neutral (a neutral IGM), but over time, their ultraviolet electromagnetic radiation ionized the hydrogen again, i.e., reionization. After reionization, the universe remained basically transparent because electrons were sufficiently far apart that the amount of scattering made little difference.
Reionization is studied by analyzing EMR from that era, including quasars (e.g., the Gunn-Peterson trough), ionized carbon fine structure line intensity mapping from its star-forming regions, the cosmic microwave background, and hopefully eventually the preceding era's 21-cm lines, and Population III stars.