Thrace (θreɪs, ; Thráki; Trakiya; Trakya) is a geographical and historical region in Southeast Europe. Bounded by the Balkan Mountains to the north, the Aegean Sea to the south, and the Black Sea to the east, it comprises present-day southeastern Bulgaria (Northern Thrace), northeastern Greece (Western Thrace), and the European part of Turkey (East Thrace), roughly the Roman Province of Thrace. Lands also inhabited by ancient Thracians extended in the north to modern-day Northern Bulgaria and Romania and to the west into Macedonia. The word Thrace, from ancient Greek Thrake (Θρᾴκη), referred originally to the Thracians (ancient Greek Thrakes Θρᾷκες), an ancient people inhabiting Southeast Europe. The name Europe (ancient Greek Εὐρώπη), also at first referred to this region, prior to the term vastly extending to refer to its modern concept. The region could have been named after the principal river there, Hebros, possibly from the Indo-European arg "white river" (the opposite of Vardar, meaning "black river"), According to an alternative theory, Hebros means "goat" in Thracian. In Turkey, it is commonly referred to as Rumeli, "Land of the Romans", which was the name traditionally given by Turkic societies to the Byzantine Empire and Orthodox Christians. In terms of ancient Greek mythology the name appears to derive from the heroine and sorceress Thrace, who was the daughter of Oceanus and Parthenope, and sister of Europa. The historical boundaries of Thrace have varied. The ancient Greeks employed the term "Thrace" to refer to all of the territory which lay north of Thessaly inhabited by the Thracians,cite book |first=Thomas |last=Swinburne Carr |title=The history and geography of Greece |publisher=Simpkin, Marshall & Company |page=56 |url= |year=1838 a region which "had no definite boundaries" and to which other regions (like Macedonia and even Scythia) were added. In one ancient Greek source, the very Earth is divided into "Asia, Libya, Europa and Thracia".