Keystone

UK: /ˈkiːstəʊn/ US: /ˈkiˌstoʊn/ ,(kē′stōn′)

WordReference Random House Learner's Dictionary of American English © 2026

key•stone /ˈkiˌstoʊn/   n. [countable]

1. Building the wedge-shaped piece at the top of an arch that holds the other pieces in place.
2. something on which associated things depend;
   basis;
   foundation:
   the keystone of one's philosophy.

WordReference Random House Unabridged Dictionary of American English © 2026

key•stone  (kē′stōn′), n. 

1. Building the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.
2. something on which associated things depend:
   the keystone of one's philosophy.
3. Sport Also called key′stone sack′. [Baseball Slang.] See second base (def. 1).

• key¹ + stone 1630–40

Synonyms

• 2. . basis, principle, foundation, linchpin.

Collins Concise English Dictionary © HarperCollins Publishers::

keystone / ˈkiːˌstəʊn/ noun

1. Also called: headstone, quoin the central stone at the top of an arch or the top stone of a dome or vault
2. something that is necessary to connect or support a number of other related things