The term Vlach is originally an exonym. All the Vlach groups used various words derived from romanus to refer to themselves: Români, Rumâni, Rumâri, Aromâni, Arumâni etc. (note: the Megleno-Romanians nowadays call themselves "Vlaşi", but historically called themselves "Rămâni"; The Istro-Romanians also have adopted the names Vlaşi, but still use Rumâni and Rumâri to refer to themselves).
The Vlachs are generally considered descendants of Romanised indigenous ancient(Paleo-Balkanic) peoples such as the Thracians (incl. Dacians) and Illyrians, as well as other populations of the Balkans such as Greeks (Hellenes and Greco-Romans).
The Vlach languages, also called the Eastern Romance languages, have a common origin from the Proto-Romanian language. Over the centuries, the Vlachs split into various Vlach groups (see Romania in the Dark Ages) and mixed with neighbouring populations: Slavs, Greeks, Albanians, Cumans, and others. (Almost all modern nations in Central and Southeastern Europe have native Vlach minorities: Hungary, Ukraine, Serbia, Croatia, Macedonia, Albania, Bosnia, Greece and Bulgaria. In other countries, the native Vlach population have been completely assimilated by the Slavic population and therefore ceased to exist: Poland, Czech Republic, Slovakia and Montenegro. Only in Romania and the Republic of Eastern Moldova, the Vlach (Dacoromanian or Romanian proper) population consist an ethnic majority today.
The word Vlach is ultimately of Germanic origin, from the word Walha, a name used by ancient Germanic peoples to refer to (mainly) Romance-speaking neighbours. As such, it shares its history with several ethnic names all across Europe, including the Welsh and Walloons. Slavic people initially used the name Vlachs when referring to Romanic people in general. Later on, the meaning became narrower or just different. For example Italy is called Włochy in Polish, and Olaszország ("Olasz country") in Hungarian.
Through history, the term "Vlach" was often used for groups which were not ethnically Vlachs, often pejoratively - for example for any shepherding community, or for Christians by Muslims (Karadjaovalides). In the Croatian region of Dalmatia, Vlaj/Vlah (sing.) and Vlaji/Vlasi (plural) are the terms used by the inhabitants of coastal towns for the people who live inland or pejoratively: barbarians who came from the mountain. In Greece, the word Βλάχος (Vláhos) is often used as a slur against any supposedly uncouth or uncultured person, but literally it simply means countryperson and is often used as a synonym for Χωριάτης (Choriátis) which means villager.
Besides the separation of some groups (Aromanians, Megleno-Romanians) during the Age of Migration, many other vlachs could be found all over the Balkans, as far north as Poland and as far west as the regions of Moravia (part of the modern Czech Republic), and the present-day Croatia where the Orthodox Vlachs took Serb identity, while the Catholic Vlachs took Croat national identity . They reached these regions in search of better pastures, and were called "Wallachians" ("Vlasi; Valaši") by the Slavic peoples.