The Beer Dictionary Entry #2

Entry 2:

DRY-HOPPING  (Brewing)

Definition: This is the process of adding hops after the wort has been cooled. This is done to cut down on the bitterness of the hops themselves. It prevents the alpha acids in the hops from isomerizing. And while it is less bitter, you still get all that great hop flavor. This is not a new technique as British brewers would sometimes throw a handful of hops into a cask of beer right before shipping to achieve the affect.