Wiktionary defines a cache as a store of things that will be required in the future, and can be retrieved rapidly. 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.
When a data entry is requested from cache and the entry exists for the given key, it is referred to as a cache hit (or simply, a hit).
When a data entry is requested from cache and the entry does not exist for the given key, it is referred to as a cache miss (or simply, a miss).
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.
The removal of entries from the cache in order to make room for newer entries (typically when the cache has run out of data storage capacity).
The removal of entries from the cache after some amount of time has passed, typically as a strategy to avoid stale data in the cache.