Hithlum was separated from Beleriand proper by the Ered Wethrin mountain chain, and was named after the sea mists which formed there at times: Hithlum is North Sindarin for "Mist-shadow"; its Quenya counterpart is Hisilómë, and its Sindarin name was either Hithlaw or Hithlû.
The Ered Wethrin ("Mountains of Shadow") formed the southern and western wall, and had only a few passes; as such they formed a natural defensive line. The eastern wall was formed by the Ered Lómin or "Echoing Mountains", which curved north-westward to Helcaraxë.
The land of Lammoth lay east of the Ered Wethrin and was not part of Beleriand or Hithlum, and the land of Nevrast was separated from Hithlum by the southern part of the Ered Lómin. Nevrast was usually seen as part of Hithlum, but its climate was that of Beleriand.
Hithlum was cold and rainy, but quite fertile. The Ñoldor landed here in the Firth of Drengist and first camped at the shores of Lake Mithrim.
Later in the First Age, Hithlum was continually under attack by Morgoth, finally being lost after the Nirnaeth Arnoediad. The Hadorians were scattered, killed, or enslaved, the Noldor were enslaved in Morgoth's mines if they could not flee in time, and Morgoth trapped the Easterlings there.
Hithlum was completely destroyed during the War of Wrath.