Powder is a loose substance made up of extremely tiny, dry, solid grains. Think of the baking powder and cocoa powder you use when you make your famous chocolate layer cake.
Powder is usually made by grinding a hard material until it's the consistency of flour, fine sand, or light snow — in fact, freshly fallen, fluffy snow is often called powder. There's a fine line between a grain and a powder, but generally powders have the characteristic of clumping together, while grains are more loose and separate. When you powder something, you apply powder to it. The Latin root, pulverem, means "dust."