A "cache miss" occurs when the data requested by a processor is not found in the cache memory. Cache memory is a small, high-speed storage area that holds frequently accessed data to speed up processing. When a cache miss happens, the system must retrieve the data from slower main memory, which can lead to delays in performance. There are different types of cache misses, including compulsory misses, which happen when data is accessed for the first time, and capacity misses, which occur when the cache is full and cannot store new data. Understanding cache misses is important for optimizing the performance of computer systems.