The "nut" (aril), ground to a fine powder. Buy powdered nutmeg from a reputable source that guarantees that the powder is not made from previously BWP (broken-wormy-punky) nuts. It's also better not to use an irradiated product. Irradiating nutmeg breaks down the fatty acids that contain the essential oils that give nutmeg its aroma and flavor. Avoid irradiated nutmeg for best quality.