Dictionary.net

The opposite of top-down design. It is nowreceived wisdom in most programming cultures that it is bestto design from higher levels of abstraction down to lower,specifying sequences of action in increasing detail until youget to actual code. Hackers often find (especially inexploratory designs that cannot be closely specified inadvance) that it works best to *build* things in the oppositeorder, by writing and testing a clean set of primitiveoperations and then knitting them together.