seating plane (1) (of a package, general)

The reference plane that designates the interface between the package or its terminals and the surface on which it is mounted.

(2) (of a surface-mounted device): The plane formed by the three terminal apexes that exhibit the greatest perpendicular distance from the package substrate, provided that the triangle formed by those three apexes encompasses the projection of the center of gravity (CoG) of the component.