Also known as bolyar; variants in other languages include Bulgarian: боляр or болярин; Ukrainian: буй[dubious ] or боярин; Russian: боя́рин, tr. boyarin, IPA: ; Romanian: boier, IPA: ( listen); and Greek: βογιάρος.
The word is likely derived from the plural form of the Bulgarian title boila ("noble"), bolyare, which is attested in Bulgar inscriptions and rendered as boilades or boliades in the Greek of Byzantine documents. Multiple different derivation theories of the word have been suggested by scholars and linguists, such as it having possible roots from old Turkic: bai ("noble, rich"; cf. "bey"), itself a derivative of beg, bag, from the Indo-European Iranic word bagh, "lord" – whence, Baghdad (lord-given), akin to Russian bog, Serbian bojh, "lord," plus Turkic är ("man, men"). The title entered Old Russian as быля (bylya, attested solely in The Tale of Igor's Campaign).
The oldest Slavic form of boyar—bolyarin, pl. bolyari (Bulgarian: болярин, pl. боляри)—dates from the 10th century, and it is found in Bulgaria, where it may have stemmed from the old Bulgar title boila, which denoted a high aristocratic status among the Bulgars. It was probably transformed through boilar or bilyar to bolyar and bolyarin. In support of this hypothesis is the 10th-century diplomatic protocol of the Byzantine Emperor Constantine VII, where the Bulgarian nobles are called boliades, while the 9th-century Bulgar sources call them boila.
A member of the nobility during the First Bulgarian Empire was called a boila, while in the Second Bulgarian Empire, the corresponding title became bolyar or bolyarin. Bolyar, as well as its predecessor, boila, was a hereditary title. The Bulgarian bolyars were divided into veliki ("great") and malki ("minor").
Presently in Bulgaria, the word bolyari is used as a nickname for the inhabitants of Veliko Tarnovo—once the capital of the Second Bulgarian Empire.
In medieval Serbia, the rank of the boyars (Боjари, bojari) was equivalent to the rank of the baron; meaning "free warrior" (or "free man" in general), it was the first rank after the non-free peasants or serfs. The etymology of the term comes from the word battle (бој, boj); the boyars of Serbia were literally "men for the battle" or the warrior class, in contrast to the peasants; they could own land but were obliged to defend it and fight for the king. With the rule of the Ottoman Empire after 1450, the Ottoman as well as the Austro-Hungarian terms exchanged the Serbian one. Today, it is an archaic term representing the aristocracy (племство, plemstvo).
From the 9th to 13th century, boyars wielded considerable power through their military support of the Kievan princes. Power and prestige of many of them, however, soon came to depend almost completely on service to the state, family history of service and, to a lesser extent, land ownership. Boyars of Kievan Rus were visually very similar to knights, but after the Mongol invasion, their cultural links were mostly lost.