Mildew is a thin white or grey living film that grows on things when the air is wet and a little warm. You see it on bathroom walls, on old cloth, on leather shoes left in a cupboard, and on plant leaves. It looks a bit like dust or powder, and it often smells stale. It is a kind of tiny living growth, not ordinary dirt you wipe once and forget. People also use mould (American spelling mold) for similar growth; though actually, gardeners and people cleaning bathrooms do not always mean the same thing by mildew and mould — the tidy dictionary splits them more neatly than people do at home. There is a verb too: cloth can mildew if you leave it wet. In everyday talk, many families just say something has gone mouldy and never reach for mildew at all.