Central processing unit
Often it makes sense to have a "cache for the cache", just as it makes sense to have a cache for RAM. In multi-level caching, there are many caches, called the L1 cache, the L2 cache, and so on. The L1 cache is the fastest (and most expensive per byte) cache and is "closest" to the CPU. The L2 cache is one step away and is slower than the L1 cache, etc. The L1 cache can often be viewed as a cache for the L2 cache, etc.