the fundamental assumptions from which something is begun or developed or calculated or explained
"the whole argument rested on a basis of conjecture"
a stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies
"The architect selected a massive granite cornerstone to be ceremonially laid on the foundation of the new library."
a stone at the outer corner of two intersecting masonry walls
"The architect carefully selected the granite cornerstone to mark where the north and south walls meet."
A stone forming the base at the corner of a building.
"The masons carefully laid the cornerstone to mark the beginning of the cathedral's construction."
In plain English: A cornerstone is something very important that forms the main part of a plan, project, or belief system.
"The new library was chosen as the cornerstone for the entire community development project."
Usage: Use this term metaphorically to describe an essential foundation or key element, but avoid applying it literally unless discussing actual masonry construction. It often appears in formal contexts when referring to critical principles or values that support a larger system.
The word comes from Middle English corner ston, a straightforward combination of "corner" and "stone." Its figurative meaning, referring to something fundamental or essential, was inspired by biblical passages such as Psalm 118:22 and several quotes in the New Testament.