cornerstone

UK: /ˈkɔː^rnə^rstəʊn/ US: /ˈkɔrnɚˌstoʊn/ ,(kôr′nər stōn′)

WordReference Random House Learner's Dictionary of American English © 2025

cor•ner•stone /ˈkɔrnɚˌstoʊn/   n. [countable]

1. Building a stone representing the starting place in the construction of a building.
2. the foundation on which something is developed:
   The speed of light as a constant is the cornerstone of Einstein's theories.

WordReference Random House Unabridged Dictionary of American English © 2025

cor•ner•stone  (kôr′nər stōn′), n. 

1. Building a stone uniting two masonry walls at an intersection.
2. Building a stone representing the nominal starting place in the construction of a monumental building, usually carved with the date and laid with appropriate ceremonies.
3. something that is essential, indispensable, or basic:
   The cornerstone of democratic government is a free press.
4. the chief foundation on which something is constructed or developed:
   The cornerstone of his argument was that all people are created equal.

• 1250–1300; Middle English; see corner, stone

Collins Concise English Dictionary © HarperCollins Publishers::

cornerstone / ˈkɔːnəˌstəʊn/ noun

1. a stone at the corner of a wall, uniting two intersecting walls; quoin
2. a stone placed at the corner of a building during a ceremony to mark the start of construction
3. a person or thing of prime importance; basis: the cornerstone of the whole argument