What is "eregion"

In the fiction of J. R. R. Tolkien, Eregion or Hollin was a realm of the Noldorin Elves in Eriador during the Second Age, located near the West Gate of Moria under the shadow of the Hithaeglir (Misty Mountains). Its capital was Ost-in-Edhil (literally Town of the Elves). This city was established by the Noldor in the year 750 of the Second Age, located where the Sirannon met the Glanduin. The elves built a large road to connect Ost-in-Edhil to the West Gate of Moria.