principal plane
/ˌprɪnsɪpl ˈpleɪn/noun
1A chief plane of reference in a body or system; especially each of three such planes (often mutually perpendicular), such as ones involving symmetry, forces, etc.
2 Optics. Either of two planes perpendicular to the optic axis of a lens such that the image of an object placed in either of these planes is the same size as the object.
Origin
Early 18th century. From principal + plane. Compare French plan principal.