(~s)
1.
A ~ 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 BRIT; in AM, use cornerstone )
The Princess of Wales laid the ~ for the extension to the Cathedral.
N-COUNT: oft with poss
2.
The ~ of something is the basic, important thing which its existence or success depends on.
...these ~s of the future: education, training, research, development.
N-COUNT: usu N of n