1. the central stone at the top of an arch that locks the other stones in place
Usage: architecture
2. something on which associated things depend for support

Examples

• The mason carefully placed the keystone to complete the stone arch.
• Without the keystone, the entire archway would collapse.
• Education is the keystone of a democratic society.
• Trust serves as the keystone of their relationship.
• The keystone species plays a crucial role in maintaining the ecosystem.
• His research became the keystone of the new theory.
• The ancient Roman arch still stands thanks to its perfectly fitted keystone.