IPA: /ˈkæp.stoʊn/
KK: /ˈkæpstoʊn/
A stone placed at the top of a structure, often symbolizing completion, or a significant achievement that represents the highest point of success.
The university's new building was the capstone of their expansion project.
Capstone is formed from "cap" (from Latin "caput", meaning head) and "stone" (from Old English "stan", meaning rock or stone). The word refers to the top stone or finishing stone that sits at the head or summit of a structure, symbolizing completion.
Think of the 'head' ('cap') of a structure being finished with a 'stone' — that's why a capstone is the final stone placed at the top.