The Free On-line Dictionary of Computing (30 December 2018):

least recently used

   (operating systems) (LRU) A rule used in a paging system
   which selects a page to be paged out if it has been used
   (read or written) less recently than any other page.  The same
   rule may also be used in a cache to select which cache entry
   to flush.

   This rule is based on temporal locality - the observation
   that, in general, the page (or cache entry) which has not been
   accessed for longest is least likely to be accessed in the
   near future.

   (1995-02-15)