Does grain equal powder or bullet?

Grain is a unit of weight, like pounds ounces or grams. There are 7000 grains to a US pound. Both bullets and powder charges are measured in grains. However, when referring to a particular version of a cartridge, grain will USUALLY refer to the weight of the bullet- as in a 125 gr. .357- they are speaking of a .357 Magnum cartridge loaded with a 125 grain bullet.