foundation stone
noun /faʊnˈdeɪʃn stəʊn/
/faʊnˈdeɪʃn stəʊn/
- a large block of stone that is put at the base of an important new public building in a special ceremony
- to lay the foundation stone of the new museum
- a basic or essential element of something
- Family life is one of the foundation stones of society.