What is a cornerstone?

Answer 1

A cornerstone can mean one of two things:

1. The stone that literally sits at the corner of a building. The cornerstone takes a lot of the pressure from the walls and gravity. Additionally, in many buildings, the cornerstone is engraved with the year in which the building was constructed.
2. An idea that forms the center or basis upon which a much larger concept or memoplex sits.
   

Answer 2

A cornerstone is a stone on the corner of a building that tells what year the building was built.