a Scots nobleman in the 11th century, nearly allied to Duncan king of Scotland.—Not contented with curbing the king's authority, he carried his pestilent ambition so far as to put him to death; and, chancing Malcolm Kenneth his son and heir into England, usurped the crown. Siward earl of Northumberland, whose daughter Duncan had married, undertook, by the order of Edward the Confessor, the protection of the fugitive prince.—He marched with an army into Scotland; defeated and killed Macbeth in Scotland; and restored Malcolm to the throne of his ancestors. Shakespeare hath made this transaction the subject of one of his best tragedies.