cornerstone
n. countable B2 Upper Intermediate US //ˈkɔɹnɝˌstoʊn// UK //kˈɔːnəstˌəʊn// cor·ner·stone
n. the most important part of something, like a plan or belief. You use this word when one thing holds everything else together.
n. the fundamental principle or feature on which a system, organization, or argument depends.
Trust is the cornerstone of our friendship.
Regular exercise is the cornerstone of her new health plan.
The rule of law remains the cornerstone of democratic governance, ensuring that no individual stands above the established legal framework.
From Middle English corner ston; equivalent to corner + stone. The figurative sense is in part after Psalm 118:22, and is also quoted several times in the New Testament.