Dry Hopping

Term for the process of adding hops during during fermentation as opposed to during boiling (wet hopping). Hops have very delicate flavor oils that are essentially destroyed during the boiling process leaving behind only the bitters. By adding hops during the cool phase a much more natural flavor and aroma is imparted into the brew.

This technique is widely used for (India) Pale Ales and was originally named by British Brewers whose practice was to add hops directly to the casks prior to shipping.

Brew Your Own has a thorough article.