Keystone (noun)

1. A central stone at the summit of an arch that locks all the stones into place and supports the weight of the structure above it.
2. A central or crucial element or part that holds a structure or system together.

Origin:

The word 'keystone' comes from the latin word 'capstone' which means 'central stone'.

Examples:

1. The keystone of the arch was carefully carved to fit into place and hold the weight of the structure above it.
2. The keystone of the company's success was its commitment to customer satisfaction.
3. The keystone species of the ecosystem play a crucial role in maintaining the balance of the environment.
4. The keystone of the argument was the evidence that was presented in court.
5. The keystone of the project was the cooperation and teamwork of all the members involved.