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Literally, a cornerstone is the first stone placed at the corner of a building, and old buildings were often built to line up starting from that one stone. Today, people use 'cornerstone' much more often in a figurative way, to mean the single most important part that everything else depends on or is built around — 'trust is the cornerstone of a good relationship', 'this policy is a cornerstone of the government's plan'. In everyday talk, this figurative sense is what you'll actually hear almost all the time.
（建築）基石、奠基石；（比喻）最重要的基礎、根本
組成corner + stone.
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
cornerstonenoun/ˈkɔː.nə.stəʊn/
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A stone forming the base at the corner of a building.
- Clutch it like a cornerstone Otherwise it all comes down Justify denials and Grip 'em to the lonesome end
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Such a stone used ceremonially, often inscribed with the architect's and owner's names, dates and other details.
- The cornerstone on the Flatiron Building is set on the Fifth Avenue facade.
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That which is prominent, fundamental, noteworthy, or central.
- Exceptional service is the cornerstone of the hospitality industry.
- That is the cornerstone of any meaningful debate about budgets and projects, regulations and policies.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。