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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 to the east of the Lower Niben, along the border with the 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 plains that form the foothills of the Jerall and Valus mountain ranges. (list of places) (Red)
Jerall Mountains — The mountain range along Cyrodiil's northern border with Skyrim. (list of places) (Pink)
Nibenay Basin — The wide valley on either side of the Niben, from Lake Rumare down to Topal Bay. (list of places) (Blue)
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 to the south of Lake Rumare between Skingrad and Bravil. (list of places) (Brown)