Oblivion: Places
Map showing the regions of Cyrodiil

There are ten regions in Cyrodiil, not including the area known as the Imperial Reserve, which seems to be merely titular:

Blackwood — The swamps of the Lower Niben bordering Elsweyr and Black Marsh. (list of places) (Dark Green)
Colovian Highlands — The mountain range on Cyrodiil's northwestern border with Hammerfell. (list of places) (Cyan)
Gold Coast — The southwestern coast of Cyrodiil along the Abecean Sea. (list of places) (Gold)
Great Forest — The woods to the north and west of the Imperial City, as far as Chorrol and the Imperial Reserve. (list of places) (Light Green)
Heartlands — The valley surrounding the upper Niben, from Lake Rumare down to Niben Bay. (list of places) (Red)
Jerall Mountains — The mountain range along Cyrodiil's northern border with Skyrim. (list of places) (Pink)
Nibenay Basin — A region of lakes and rivers lying east of the Niben, between the Heartlands and the Valus Mountains. (list of places) (Purple)
Nibenay Valley — A small region around the southern end of Niben Bay. (list of places) (Yellow)
Valus Mountains — The mountain range on Cyrodiil's northeastern border with Morrowind. (list of places) (White)
West Weald — The grasslands southwest of Lake Rumare between Kvatch and Bravil. (list of places) (Brown)