Loft is the measure of the fluffiness of Down. Loft specifically refers to the number of cubic inches that one ounce of down will fill. This measurement is done under and with specific, internationally accepted specifications and methods. The test is conducted in a Plexiglas cylinder under exacting conditions of temperature and humidity. The larger, more mature the bird, the higher the loft of its down, when harvested.
Good quality products start with a loft factor of the down at about 550 cubic inches per ounce. The fluffier the down or the higher the loft, the warmer it will keep you, and the less it takes to fill a duvet or pillow. Therefore a high quality product will be lighter and still very warm and / or resilient.
Loft of down can be anywhere from about 300 to 800 cubic inches per ounce. Loft of down is probably the largest factor in determining the price of a down product. Generally higher loft products will also last much longer as the down has grown closer to full maturity and its filaments are much stronger. Lower quality loft products have weak and brittle down and will require replacing much sooner.
Check out this infographic, The Lowdown On Loft!