Cornerstone

Alternative forms

 * corner stone

Noun

 * 1) A ceremonial stone set at the corner of a building, joining two exterior walls, and often inscribed with the starting and completion dates of construction, the name of the architect and owner, and other details.
 * 2) By extension, that which is prominent, fundamental, noteworthy, or central.

Thesaurus
bench mark, cardinal point, chief thing, climax, core, crisis, critical point, crux, essence, essential, essential matter, first stone, footstone, foundation stone, fundamental, gist, gravamen, great point, headstone, heart, high point, important thing, issue, kernel, keystone, landmark, main point, main thing, material point, meat, milestone, nub, pith, pivot, quoin, real issue, salient point, sine qua non, substance, substantive point, the bottom line, the point, turning point