A foundation stone is a large block of stone built into a large public building near the bottom. It is often involved in a ceremony for the opening of the building, and has writing on it recording this.
[mainly British]
The princess laid the foundation stone for the extension to the cathedral.
regional note: in AM, use cornerstone
2. countable noun
The foundation stoneof something is the basic, important thing which its existence or success depends on.
...these foundation stones of the future: education, training, research, development. [+ of]
foundation stone in British English
noun
a stone laid at a ceremony to mark the foundation of a new building