A cache is a collection of temporary data that either duplicates data located elsewhere or is the result of a computation. Data that is already in the cache can be repeatedly accessed with minimal costs in terms of time and resources.

A cache entry consists of a key and its mapped data value within the cache.

When a data element is requested from cache and the element exists for the given key, it is referred to as a cache hit (or simply, "a hit").

When a data element is requested from cache and the element does not exist for the given key, it is referred to as a cache miss (or simply, "a miss").

When entries are removed from the cache in order to make room for newer entries (typically when the cache has run out of data storage capacity), it is referred to as eviction.

When entries are removed from the cache after some defined amount of time has passed, it is referred to as expiration.

Data that has recently been used by an application is very likely to be accessed again soon. Such data is considered hot. A cache may attempt to keep the hottest data most quickly available, while attempting to choose the least hot data for eviction.

The system-of-record is the authoritative source of truth for the data. The cache acts as a local copy of data retrieved from or stored to the system-of-record (SOR). The SOR is often a traditional database, although it might be a specialized file system or some other reliable long-term storage. It can also be a conceptual component such as an expensive computation.