"cornerstone" in the noun sense
1. basis, base, foundation, fundament, groundwork, cornerstone
the fundamental assumptions from which something is begun or developed or calculated or explained
"the whole argument rested on a basis of conjecture"
2. cornerstone
a stone in the exterior of a large and important building usually carved with a date and laid with appropriate ceremonies
3. cornerstone
a stone at the outer corner of two intersecting masonry walls
Source: WordNet® (An amazing lexical database of English)
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