Definition of "boyar"

Boris had abdicated in 889, leaving the throne to his son Vladimir, who had immediately identified himself with the boyar aristocracy which Boris had done his utmost to crush.

1997, John Julius Norwich, A Short History of Byzantium, Penguin, published 1998, page 159

A long series of wars was fought in the sixteenth century to keep Polish influence at bay in the West Russian lands, and prevent it from seducing Muscovy's restless boyars, the warrior-barons whose independence the grand dukes were determined to crush.

2007, John Darwin, After Tamerlane, Penguin, published 2008, page 68

some of his family looked to Orthodox Christianity to sustain them, and not only many of his boyars but most of his subjects were Orthodox Christians.

2009, Diarmaid MacCulloch, A History of Christianity, Penguin, published 2010, page 514