Colovia is the historical name for the region of Tamriel that lies between the Niben River and the Abecean Sea. Colovia is a part of the modern-day province of Cyrodiil, but is culturally distinct from the areas of eastern Cyrodiil, known as Nibenay. Although currently unified with the rest of Cyrodiil, it used to be a separate nation.
Geographically, the region most frequently identified as Colovia is bordered to the west by the Abecean Sea, to the east by Lake Rumare and the Niben River, to the south by the Strid River and Valenwood Forest, and to the north by the Brena River and Jerall Mountains. Politically, the region is also bounded by its borders with Hammerfell to the northwest, and Elsweyr to the southeast. Colovia's geography can be broken down into three primary regions: the Gold Coast, the West Weald, and the Colovian Highlands.
The Gold Coast is the western-most region of Colovia, consists of the entire coastline on the Abecean Sea, and continuing inland until the start of the West Weald forest and foothills of the Colovian Highlands. The borders of the Gold Coast follow the coastline inward to the mouths of the Strid River to the south, and the Brena River to the north. It is the flattest region in the entire Cyrodiilic province, and much less densely foliated than regions further east. The region gets its name from the sparse golden grass that dots the coastline, alongside the occasional stand of evergreen trees. The Gold Coast also contains Cyrodiil's primary port town of Anvil.
The West Weald is a large, forested region that runs along the Strid River, abutting the border with Valenwood, and continues in a roughly half-moon shape into the hills of the Colovian Highlands and Jerall Mountains. Once the Strid River ends, the West Weald dips into a steep valley that follows the border with Elsweyr east into the river basin surrounding the Nibenay River and its western tributaries. To the northeast, the forest melds with the Great Forest that surrounds Lake Rumare and the Imperial City. The area is mildly hilly, with much of the bedrock showing through, and contains a number of lakes.
The West Weald contains a wealth of flowering plants and deciduous forests, as well as a good amount of quality farmland. In particular, the rolling hills make ideal locations for grapes, and both the Tamika Vineyards and Surilie Brothers Vineyards are found just outside Skingrad, near the head of the Strid River. The forested hills also support abundant wildlife (some rather dangerous), and a variety of flowing plants.
The Colovian Highlands are a sparsely populated region making up the western foothills of the Jerall Mountains. It cuts a fairly straight swathe of land running northeast from the coast, along the Hammerfell border, into the Jerall Mountains proper. The vegetation is similar to that of the Gold Coast, though more dense, and there is a higher population of wild animals (in particular, mountain lions and wolves). There are almost no settlements, human or otherwise, in the region, though the city of Chorrol was founded on the edge of the foothills in the Great Forest. A single road runs past Chorrol into the hills, and access to the few ruins or dungeons typically requires following extensive and circuitous paths along the mountainside.
During the Merethic Era, Colovia was part of the Ayleid Empire in central Tamriel. When Alessia overthrew the Ayleid and founded Cyrodiil, it did not initially include the regions west of the Nibenay Basin. These regions (which included the Ayleid capital) banded together to form a loosely organized kingdom, under the name The Colovian Estates. The region would remain essentially independent until well into the First Era.
The Cyrodiilic Empire had a fairly stable relationship with Colovia, an arrangement which benefited both regions. Colovia held one of only two ports that gave Cyrodiil access to ocean trade, the other going through the swampy Blackwood, dangerously close to both Elsweyr and the Black Marsh. Most of Colovia was not well suited for farming, especially large-scale crops and livestock, but was a prime source of furs and pelts from its mountain animals, as well as the finest wines in Tamriel.
In the early days of the Empire, its Heartland was often the target of ambitions of mer rulers, most notably the Direnni Elves of High Rock. Colovia was situated between Cyrodiil and its enemies, and saw significant military action as armies clashed on its soil. It was also the major focus of Cyrodiilic expansion under the First Dynasty, which was constantly pushing the border of Cyrodiil west. In 1E 1029, the Empress "officially" pushed Cyrodiil's border all the way to High Rock, which would have included at least the entirety of the Colovian Highlands. Finally, in 1E 2703, when the Tsaesci of Akavir invaded Tamriel, the Colovian Estates was fully integrated into the Cyrodiilic Empire under King Reman I, beginning the Second Dynasty of Cyrodiilic rule.
Also, as an interesting side note, the Colovian Estates were given to Imperial General Talos to rule, by then-Emperor Cuhlecain, marking the start of Talos rise to power. Talos, of course, later became Emperor himself, unified the entire continent, and took the Cyrodiilic name Tiber Septim.