Timok River in the context of "Discharge (hydrology)"

Northern Bulgaria (Bulgarian: Северна България, romanized: Severna Bylgarija), also called Moesia (Bulgarian: Мизия, Mizija) is the northern half of Bulgaria, located to the north of the main ridge of the Balkan Mountains which conventionally separates the country into a northern and a southern part. Besides the Balkan Mountains, Northern Bulgaria borders the Timok River and Serbia to the west, the Danube River and Romania to the north, and the Bulgarian Black Sea Coast to the east.

Geographically, the region's terrain is relatively uniform, dominated by the hilly Danubian Plain, with some low plateaus to the east. Northern Bulgaria covers an area of 48,596 square kilometres and has a population of 2,674,347 according to the 2011 census (36% of Bulgaria's entire population), with a population density of 55 people per km². The three largest cities are Varna, Rousse and Pleven.

The Danubian Plain (Bulgarian: Дунавска равнина, romanized: Dunavska ravnina) constitutes the northern part of Bulgaria, situated north of the Balkan Mountains and south of the Danube. Its western border is the Timok River, and to the east it borders the Black Sea. The plain has an area of 31,523 square kilometres (12,171 sq mi). It is about 500 kilometres (310 mi) long and 20 to 120 kilometres (12 to 75 mi) wide.

The Danubian Plain is contiguous with the Wallachian Plain (forming the Lower Danubian Plain), but its elevation is slightly higher and the relief is more hilly and rolling, featuring numerous plateaux and river valleys. The climate is markedly temperate continental with a weak Black Sea influence in the east. Precipitation is on average 450–650 mm a year. Important rivers include the Danube, the Iskar, the Yantra, the Osam, the Vit, the Rusenski Lom, the Ogosta, and the Lom.

In Roman literature of the early 1st century CE, the Moesi (/ˈmiːsaɪ/ or /ˈmiːzaɪ/; Ancient Greek: Μοισοί, Moisoí or Μυσοί, Mysoí; Latin: Moesi or Moesae) appear as a tribe who lived in the region around the Timok River to the south of the Danube. The Moesi do not appear in ancient sources before Augustus's death in 14 CE and are mentioned only by three authors dealing with the Roman warfare in the region and the ethnonymic situation between mid-1st century BC and mid-1st century CE: Ovid, Strabo and Livy. A Paleo-Balkan tribe known as the Moesi never actually existed in the Danube area before that period, it was a Roman invention. The ethnonym was transplanted from Asia Minor Mysians to the Balkans by the Romans as a replacement of the name of the Dardani who lived in the territory that later became the province of Moesia Superior. This decision in Roman literature is linked to the appropriation of the name Dardani in official Roman ideological discourse as Trojan ancestors of the Romans and the creation of a fictive name for the actual Dardani who were seen as barbarians and antagonists of Rome in antiquity. This new fictive Augustan terminology was illogically and controversially argumented by Strabo as the result of Aelius Catus's displacement of 50,000 Getae from the north to the south of the Danube, who settled areas in the north-eastern parts of the later province of Moesia Superior, thereafter being called "Moesi".

The Latin name Moesia was given first to the province of Moesia Superior and expanded into Moesia Inferior along the Danube. After the recreation of Dardania, Moesia referred to Moesia Prima, the northern part of Moesia Superior. A civitas of the Moesi which was reorganized as a Roman colony was located around Ratiaria in the first century AD.

The Timočka Krajina (Serbian: Тимочка Крајина, lit. 'Timok Frontier') or Timok Valley (Romanian: Valea Timocului), is a geographical region in eastern Serbia around the Timok River. It roughly corresponds to two administrative districts, Bor and Zaječar, which have a combined population of 197,815.