foundation stone 
noun
1A stone laid at a ceremony to celebrate the beginning of construction of a building.Construction of the tower began on 11 December 1912, with the laying of a foundation stone, in a ceremony held on the completed foundations and steps....
- In October 2002, Mr Callely laid the foundation stone of the building, which received €25 million in Department of Health funding.
- It was July 21, 1962 when the Governor of Western Australia, Sir Charles Gairdner, attended a ceremony to lay the foundation stone.
1.1A basic or essential element of something: family life is one of the foundation stones of a good society...
- It is an essential foundation stone for any serious appreciation of the cultural history of a large proportion of the world.
- Now that means that a very fundamental foundation stone to the law of defamation is in fact non-existent.
- There are two centres, two foundation stones to my life here.