In antiquity, the Roman province of Dalmatia was much larger than the present-day Split-Dalmatia County, stretching from Istria in the north to modern-day Albania in the south. Dalmatia signified not only a geographical unit, but was an entity based on common culture and settlement types, a common narrow eastern Adriatic coastal belt, Mediterranean climate, sclerophyllous vegetation of the Illyrian province and Adriatic carbonate platform
Today, Dalmatia is a historical region only, not formally instituted in Croatian law. Its exact extent is therefore uncertain and subject to public perception. According to Lena Mirošević and Josip Faričić of the University of Zadar:

...the modern perception of Dalmatia is mainly based on the territorial extent of the Austrian Kingdom of Dalmatia, with the exception of Rab, which is geographically related to the Kvarner area and functionally to the Littoral–Gorski Kotar area, and with the exception of the Bay of Kotor, which was annexed to another state (Montenegro) after World War I. Simultaneously, the southern part of Lika and upper Pounje, which were not part of Austrian Dalmatia, became part of Zadar County. From the present-day administrative and territorial point of view, Dalmatia comprises the four Croatian littoral counties with seats in Zadar, Šibenik, Split, and Dubrovnik.

"Dalmatia" is therefore generally perceived to extend approximately to the borders of the Austrian Kingdom of Dalmatia. However, due to territorial and administrative changes over the past century, the perception can be seen to have altered somewhat with regard to certain areas, and sources conflict as to their being part of the region in modern times:
• The Bay of Kotor area in Montenegro. With the subdivision of the Kingdom of Yugoslavia into oblasts in 1922, the whole of the Bay of Kotor from Sutorina to Sutomore was granted to the Zeta Oblast, so that the border of Dalmatia was formed at that point by the southern border of the former Republic of Ragusa. The Encyclopædia Britannica defines Dalmatia as extending "to the narrows of Kotor" (i.e. the southernmost tip of continental Croatia, the Prevlaka peninsula). Other sources, however, such as the Treccani encyclopedia and the Rough Guide to Croatia still include the Bay as being part of the region.
• The island of Rab, along with the small islands of Sveti Grgur and Goli Otok, were a part of the Kingdom of Dalmatia and are historically and culturally related to the region, but are today associated more with the Croatian Littoral, due to geographical vicinity and administrative expediency.
• Gračac municipality and northern Pag. A number of sources express the view that "from the modern-day administrative point of view", the extent of Dalmatia equates to the four southernmost counties of Croatia: Zadar,

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The regional name Dalmatia originates from Dalmatae, the name of the Illyrian, Balkan tribe who were the original inhabitants of the region, and from which the later toponym, Delminium, is derived. It is considered by various academics to be connected to the Albanian dele and its variants which include the Gheg form delmë, meaning "sheep", and to the Albanian term delmer, "shepherd". Vladimir Orel argues that the Gheg form delmë hardly has anything in common with the name of Dalmatia because it represents a variant of dele with *-mā, which is ultimately from proto-Albanian *dailā. According to Danilo Savić, an argument against that connection is the lack of compelling evidence in ancient literary sources that Delmatae is derived from a word meaning "sheep". The ancient name Dalmana, derived from the same root, testifies to the advance of the Illyrians into the middle Vardar, between the ancient towns of Bylazora and Stobi. The medieval Slavic toponym Ovče Pole ("plain of sheep" in South Slavic) in the nearby region represents a related later development. According to István Schütz, in Albania, Delvinë represents a toponym linked to the root *dele.

The form of the regional name Dalmatia and the respective tribal name Dalmatae are later variants as is already noted by Appian (2nd century AD). His contemporary grammarian Velius Longus highlights in his treatise about orthography that the correct form of Dalmatia is Delmatia, and notes that Marcus Terentius Varro who lived about two centuries prior to Appian and Velius Longius, used the form Delmatia as it corresponded to the chief settlement of the tribe, Delminium. The toponym Duvno is a derivation from Delminium in Croatian via an intermediate form *Delminio in late antiquity. Its Latin form Dalmatia gave rise to its current English name. In the Venetian language, once dominant in the area, it is spelled Dalmàssia, and in modern Italian Dalmazia. The modern Croatian spelling is Dalmacija, and the modern Serbian Cyrillic spelling is Далмација (pronounced [dǎlmaːt͡sija]).

Dalmatia's name is derived from the name of an Illyrian tribe called the Dalmatae who lived in the area of the eastern Adriatic coast in the 1st millennium BC. It was part of the Illyrian Kingdom between the 4th century BC and the Illyrian Wars (220, 168 BC) when the Roman Republic established its protectorate south of the river Neretva. The name "Dalmatia" was in use probably from the second half of the 2nd century BC and certainly from the first half of the 1st century BC, defining a coastal area of the eastern Adriatic between the Krka and Neretva rivers. It was slowly incorporated into Roman possessions until the Roman province of Illyricum was formally established around 32–27 BC. In 9 AD, the Dalmatians raised the last in a series of revolts together with the Pannonians, but it was finally crushed and, in 10 AD, Illyricum was split into two provinces, Pannonia and Dalmatia, which spread into larger area inland to cover all of the Dinaric Alps and most of the eastern Adriatic coast.

The historian Theodor Mommsen wrote in his book, The Provinces of the Roman Empire, that all Dalmatia was fully romanized by the 4th century AD. However, analysis of archaeological material from that period has shown that the process of Romanization was rather selective. While urban centers, both coastal and inland, were almost completely romanized, the situation in the countryside was completely different. Despite the Illyrians being subject to a strong process of acculturation, they continued to speak their native language, worship their own gods and traditions, and follow their own social-political tribal organization which was adapted to Roman administration and political structure only in some necessities.

The fall of the Western Roman Empire, with the beginning of the Migration Period, left the region subject to Gothic rulers Odoacer and Theodoric the Great. They ruled Dalmatia from 480 to 535 AD, when it was restored to the Eastern Roman (Byzantine) Empire by Justinian I.
In the Early Middle Ages, the territory of the Byzantine province of Dalmatia reached in the North up to the river Sava, and was part of the Praetorian prefecture of Illyricum. In the middle of the 6th and the beginning of the 7th century began the Slavic migration, which caused the Romance-speaking population, descendants of Romans and Illyrians (speaking Dalmatian), to flee to the coast and islands. The hinterland, semi-depopulated by the Barbarian Invasions, Slavic tribes settled. The Slavs alongside Avars by 619 brought to ruin the capital Salona (an event that allowed for the settlement of the nearby Diocletian's Palace in Spalatum), Asseria, Varvaria, Burnum, Scardona, Epidaurum and Acruvium (resulting with the foundation of Kotor), and Epidaurum (resulting with the foundation of Ragusa). The arrived tribes of Croats, Serbs and other Slavs founded sclavi…

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