1.
foundation stone, cornerstone
Originally the stone placed at the base of a building to support its structure. Now primarily used figuratively to mean the fundamental basis or groundwork upon which something important is built.
Usage
A literary and elevated word for foundation or basis. Almost always used figuratively in modern Japanese. The literal meaning (a foundation stone in construction) is rare outside of historical contexts.
Compare with 基盤 (base, infrastructure — more concrete and practical) and 土台 (foundation — can be literal or figurative, less formal).