1A formal agreement or treaty made between royal or noble families, or between members of the same royal or noble family. Now chiefly with lower-case initials.
2Specifically. Any one of a series of three treaties made in the 18th century between the Bourbon dynasties of France and Spain and their allies for common action, especially against Britain and Austria; an alliance resulting from one of these treaties.
3(The name given to) a small closed group of Tory politicians and officials who exercised extensive political power in Upper Canada in the first part of the 19th century. Hence (with lower-case initials): any political clique.
Mid 18th century. From family + compact, after French pacte de famille family alliance, Spanish pacto de familia, or Italian patto di famiglia family alliance.