leading aristocrat; great noble; magnate
Refers to a high-ranking noble or a powerful aristocratic family. Can be used historically or in fantasy settings. 'Boyar' is a specific historical Russian term; use only in that context.
He is a descendant of a medieval great noble.
In that country, the leading aristocrats controlled politics.
Compound of 大 (dai, 'great') and 貴族 (kizoku, 'noble, aristocrat'). The exact historical derivation is uncertain, but the term follows standard Sino-Japanese word formation.