Zachlumia or Zachumlia (Захумље, zǎxuːmʎe), also Hum, was a medieval principality located in the modern-day regions of Herzegovina and southern Dalmatia (today parts of Bosnia and Herzegovina and Croatia, respectively). In some periods it was a fully independent or semi-independent South Slavic principality. It maintained relations with various foreign and neighbouring powers (Byzantine Empire, First Bulgarian Empire, Kingdom of Croatia, Principality of Serbia) and later was subjected (temporarily or for a longer period) to Kingdom of Hungary, Kingdom of Serbia, Kingdom of Bosnia, Duchy of Saint Sava and at the end to the Ottoman Empire.
Zachlumia is a derivative of Hum, from Proto-Slavic *xŭlmŭ, borrowed from a Germanic language (cf. Proto-Germanic *hulma-), meaning "Hill". South Slavic Zahumlje is named after the mountain of Hum (za + Hum "behind the Hum"), above Bona, at the mouth of the Buna. The principality is named Zahumlje or Hum in Serbo-Croatian (Serbian Cyrillic: Захумље, Хум). It is Zachlumia in Latin, Хлъмъ in Old Church Slavonic, and Ζαχλούμων χώρα ("land of Zachlumians") in Greek. The names Chelmania, Chulmia and terra de Chelmo appear in later Latin and Italian chronicles.
De Administrando Imperio described the polity of Zachlumia, likely during the reign of Časlav Klonimirović (r. 927–960): "From Ragusa begins the domain of the Zachloumoi (Ζαχλοῦμοι) and stretches along as far as the river Orontius, and on the side of the coast it is neighbour to the Pagani, but on the side of the mountain country it is neighbour to the Croats on the north and Serbia at the front ... The Zachloumoi that now live there are Serbs, originating from the time of the prince (archon) who fled to emperor Heraclius ... The land of the Zachloumoi comprises the following cities: Ston (το Σταγνον / to Stagnon), Mokriskik (το Μοκρισκικ), Josli (το Ιοσλε / to Iosle), Galumainik (το Γαλυμαενικ / to Galumaenik), Dobriskik (το Δοβρισκικ / to Dovriskik)".