Transdermal means absorbed through the skin — describing substances, usually medicines, that pass through the skin into the bloodstream. Nicotine patches, hormone replacement patches, and some pain-relief patches all use the transdermal route, which delivers a steady dose over hours or days while avoiding the digestive system and first-pass metabolism in the liver. The skin is a formidable barrier, so transdermal delivery suits only small, fat-soluble molecules, often helped by penetration enhancers. In nutrition the concept appears in discussions of whether nutrients such as magnesium can be meaningfully absorbed from bath salts or creams — the evidence for significant uptake is weak. The skin keeps most things out by design.