Source : Free On-Line Dictionary of Computing

bottom-up implementation
     
         The opposite of {top-down design}.  It is now
        received wisdom in most programming cultures that it is best
        to design from higher levels of abstraction down to lower,
        specifying sequences of action in increasing detail until you
        get to actual code.  Hackers often find (especially in
        exploratory designs that cannot be closely specified in
        advance) that it works best to *build* things in the opposite
        order, by writing and testing a clean set of primitive
        operations and then knitting them together.
     
        [{Jargon File}]
     
        (1996-05-10)