A keystone is a central stone at the summit of an arch, locking the other stones in place. It is crucial for the stability of the arch, as it distributes weight evenly and prevents the structure from collapsing. The design of the keystone allows for the arch to bear heavy loads, making it a fundamental element in architecture. In a broader context, the term "keystone" can also refer to a key component in various systems or processes. For example, in ecology, a keystone species is one that has a disproportionately large effect on its environment relative to its abundance, helping to maintain the structure of the ecosystem.