a special type of inferior good where demand increases when price increases

A good for which an increase in the price raises the quantity demanded. This is a special case where there is a direct relationship between changes in the price of a good and changes in that good's quantity demanded.

A Giffen good is a product for which a rise in price of this product makes people buy even more of the product. Giffen goods may or may not exist in the real world, but there is an economic model that explains how such a thing could exist. Giffen goods are named after Sir Robert Giffen, who was attributed as the author of this idea by Alfred Marshall in his book Principles of Economics.