Dry hop, dry hopping, or dry hopped beer is beer that has had hops added to it during fermentation as a way of increasing hop aroma. Dry hopping is typically conducted in secondary fermentation or after primary fermentation has completed to help assure that the aroma stays in the fermenter as opposed to being pushed through the airlock with the escaping CO2.
When dry hopping, little to no bitterness is added to the fermenting beer, as the alpha acid resin is relatively insoluble in a fermenting beer at that temperature. The process of dry hopping typically lasts anywhere from a few days to a couple of weeks. Some brewers report that their beer acquires a grassy flavor if it is allowed to dry hop for more than a week or so. If grassy off flavors are a concern, you can always add additional hops for a shorter duration of time to achieve the desired dry hop aroma.
Dry hopping in a secondary fermentation keg. Whole hops are used in this case, but pellet or plug hops may also be used.