皮肤(pí fū) - skin in Mandarin

皮肤 (pí fū) is skin, specifically human skin. It is the word for a dermatologist's sign, a cosmetics advert and a doctor's question alike, and it is uncountable in practice: you talk about 皮肤 the way English talks about skin, not about skins.

The related words split by what is wearing the skin. 皮 alone means hide or leather and is the one you want for animals and for the peel of fruit, so 香蕉皮 is a banana skin. 皮肤病 is a skin disease and 皮肤科 the dermatology department, both common enough to be worth memorising whole.

The trap is reaching for 皮肤 whenever English says skin. A banana skin, a leather bag and the skin on hot milk are all 皮, never 皮肤. Keep 皮肤 for the living surface of a person, and let 皮 handle everything else.