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Caching is a term used in computer science. The idea behind a cache (pronounced "cash" /ˈkæʃ/ KASH ) is very simple: Very often, obtaining a result for a calculation is very time-consuming, so storing the result is generally a good idea. Two kinds of storage media are used: One is usually quite big, but accessing it is "slow"; the other one can be accessed much faster, but generally it is small. The very basic idea behind caching is to use the medium that is fast to access to have copies of data. There is no difference between the copy, and the original. Accessing the original data may take a long time, or it may be expensive to do (for example: the results of a difficult problem that take a long time to solve). For this reason, it is much "cheaper" to simply use the copy of the data from the cache. Put differently, a cache is a temporary storage area that has copies of data that is used often. When a copy of the data is in this cache, it is faster to use this copy rather than re-fetching or re-calculating the original data. This will make the average time needed to access the data shorter. Putting a new value into a cache often means that an older value needs to be replaced. There are different ideas (usually called "strategies") on how to select the value to replace.