a stone uniting two masonry walls at an intersection.

a stone representing the nominal starting place in the construction of a monumental building, usually carved with the date and laid with appropriate ceremonies.

something that is essential, indispensable, or basic

The cornerstone of democratic government is a free press.

the chief foundation on which something is constructed or developed

The cornerstone of his argument was that all people are created equal.

noun

anchor , base , essential , foundation , key element , keystone , linchpin , main ingredient , mainspring , mainstay , pillar , fundament , fundamental , root , rudiment , basis , coigne , marking , quoin , support