A boyar, or bolyar (Bulgarian: боляр or болярин, Ukrainian: буй or боярин, Russian: боярин, Romanian: boier pronounced [boˈjer] ( listen), Greek: βογιάρος), was a member of the highest rank of the feudal Bulgarian, Moscovian, Kievan Rus'ian, Wallachian, and Moldavian aristocracies, second only to the ruling princes (in Bulgaria, tsars), from the 10th century through the 17th century. The rank has lived on as a surname in Russia and Finland, where it is spelled Pajari.

According to most sources, the word is of Turkic origin. Some believe that it is composed of the Turkic roots bai ("noble, rich") and är. Another possibility is that the word originated from the Bulgarian title boila ("noble"), which is attested in Bulgar inscriptions and rendered as boilades or boliades in the Greek of Byzantine documents. This title certainly did enter Old Russian as быля (bylya).

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.