Greek akrogōniaios (Ephesians 2:20, 1 Peter 2:6), translating Hebrew pinnah. In ancient masonry the cornerstone was the first stone laid at the building's corner; every other stone was measured by it. It had to be cut perfectly square, set perfectly plumb, and sized to carry the load. A flawed cornerstone guaranteed a crooked building; the right cornerstone defined every line. Psalm 118:22 — "The stone that the builders rejected has become the cornerstone" — is quoted by Jesus (Matthew 21:42), Peter (Acts 4:11, 1 Peter 2:7), and Paul (Ephesians 2:20).
To call Christ the Chief Cornerstone is to say He defines the shape of the Church. The foundation is the apostles and prophets; the cornerstone is Christ Jesus Himself (Ephesians 2:20). Every living stone — every believer (1 Peter 2:5) — is measured and placed with reference to Him. Doctrines that don't line up with Christ don't get laid into the building. He is simultaneously the rejected stone (the builders — religious leaders — threw Him out) and the indispensable stone (God made Him the cornerstone anyway); He is the stone sinners stumble over (1 Peter 2:8) and the stone God places as a sure foundation for faith. Modern churches tempted to build on sociology, marketing, or cultural relevance discover their walls won't plumb — because they've swapped out the cornerstone.