Cornerstone (noun)

1. A stone that is placed at a corner of a building or structure and acts as a foundation.
2. Something or someone that is the foundation or basis of something else.

Origin:

From corner + stone.

Examples:

1. The cornerstone of the building was laid in a ceremony attended by dignitaries.
2. The cornerstone of their success was the teamwork and mutual support they shared.
3. The cornerstone of his political platform was the promotion of social justice.
4. The cornerstone of the company's values was to always put the customer first.
5. The cornerstone of the financial plan was to invest in high-yielding stocks.