基石 (jī shí) literally means 'foundation stone' — 基 (jī) means 'foundation, base, or fundamental,' and 石 (shí) means 'stone.' Together, they form a metaphorical noun referring to something essential that supports or underpins a larger structure, system, or idea. Like its English equivalents 'cornerstone' or 'keystone,' 基石 is rarely used to describe an actual physical stone in modern speech; instead, it appears in formal, literary, or rhetorical contexts to emphasize foundational importance.

You’ll commonly encounter 基石 in political speeches, educational policy documents, historical narratives, or discussions about values — for example, 'democracy is the cornerstone of modern society' or 'trust is the cornerstone of any healthy relationship.' It carries positive, weighty connotations and implies stability, indispensability, and long-term significance. Because of its elevated register, it’s seldom used in casual conversation or with concrete, everyday objects.