speed of light

   The absolutely fastest a particular algorithm or application could be
   implemented,  given  a  set  of  constraints  that  are assumed to be
   unchangeable.  For  example, "This would take 60 microseconds without
   any processing whatsoever, so that's the speed of light." However, as
   one  brilliant  hacker  once  commented:  "Remember that the speed of
   light only is constant if you can't redesign the universe."

[glossary]