Source : Free On-Line Dictionary of Computing

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)