Riboflavin is vitamin B2: a substance your body needs in small amounts, found in meat, milk, eggs, and some vegetables, and often added to breakfast cereal. It helps the body turn food into energy and keeps skin and eyes in ordinary working order. You meet the word on food labels, in biology class, and at the chemist, not in casual chat — at the table people just say vitamin B2. It is a yellowish substance (the name’s second half is about that colour), which is why some vitamin tablets leave a bright yellow colour in urine; that is normal, not a disease. A real lack of it is uncommon where diets include milk or meat, but it can show as sore cracks at the corners of the mouth. Though actually, “B2” and “riboflavin” are the same thing, not two different vitamins.

核黃素（維他命B2。台灣口語常說維他命，較少說維生素。）