A keystone is the central stone at the top of an arch that locks the other stones in place. It can also mean the most important part of a system or idea that holds everything else together.
Formed in English from “key” and “stone”, based on the idea that this stone is the “key” to the arch’s strength. The figurative sense grew from this engineering image.
An arch without a keystone collapses, even if every other stone is perfect. In ecosystems, “keystone species” play the same role—remove them and the whole system can fall apart. The word teaches a quiet lesson: sometimes the most important piece isn’t the biggest, just the most connected.
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