The "write-invalidate" protocol is a method used in computer systems to maintain data consistency across multiple caches. When one cache writes new data, it sends an invalidate signal to other caches that may hold the same data. This ensures that outdated or stale data is not used, preventing errors in processing. This protocol is commonly used in multiprocessor systems where multiple CPUs access shared memory. By invalidating the data in other caches, the system ensures that all processors work with the most current information, enhancing overall performance and reliability in applications like distributed computing and real-time systems.