Caries is the slow decay that eats into a tooth. Dentists say dental caries; it is their technical name for what most people call tooth decay or a cavity. The word was borrowed from Latin. In English it behaves like a mass word in medical talk: dentists speak of caries as the disease, not as “one carie”. Though actually everyday English almost never uses it. A patient says “I've got a cavity” or talks about a bad tooth; caries lives on clinic forms and in textbooks. If you meet it, you are probably reading something written for dentists or for health class.