Least Recently Used replacement algorithm
The Least Recently Used replacement algorithm (usually abbreviated as LRU) is an algorithm for choosing which entry to 'recycle' first in an array of resources, when a need arises for a free resource, and they are already all in use. It says, as the name implies, that one should recycle the entry which was most recently used the longest time ago, since the ones used since then are more likely to be used again soon.
The reasoning behind minimizing the number of times entries are freed is that freeing a resource likely involves some effort (e.g. writing a modified disk block from a buffer back onto secondary storage), so that effort should be expended are rarely as possible.
Providing the information needed to determine which entry is the least recently used entry may require some effort, in both hardware and software systems. There are thus algorithms which approximate 'true' LRU, but use less overhead.