taper: to become smaller or thinner toward one end
When used in context with WRX/STi exhaust piping, the taper refers to the part of the downpipe where the pipe gradually gets smaller toward the end where the flange connects to the cat back. This makes it possible for a larger 3" downpipe to make a smooth transition to a smaller 2.5" or 2.25" cat back pipe (A good example of this is if you wanted to use a larger aftermarket downpipe with your stock cat back so you would have more power and still maintain a stock look from the back of your car. Cobb's 3" downpipe has a slight taper toward the end of it so you could successfully connect your stock cat back to it if you wanted to.). The exhaust flows a lot smoother this way rather than if there was no taper and the larger 3" pipe suddenly became a smaller 2.5" or 2.25" pipe. That would be like hitting a wall and create all kinds of turbulence and rob you of some precious horsepower. So, tapers are good unless you're going from a 3" downpipe to a 3" cat back pipe - in which case, you don't need a taper at all because the pipes are the same size. Get it?
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)