A "capacity miss" occurs in computer systems when the cache, a small and fast storage area, cannot hold all the data needed for processing. When the data required is not found in the cache, the system must retrieve it from slower main memory, leading to delays in performance. This situation typically arises when the working set of data exceeds the cache size. To minimize capacity misses, system designers often increase the cache size or optimize data access patterns. Techniques like cache replacement policies can also help manage which data remains in the cache, aiming to keep frequently accessed data readily available and reduce the likelihood of capacity misses.