Original: For example, ascorbic acid (vitamin C) is a vitamin for humans, but not for most other animals, and biotin and vitamin D are required in the human diet only in certain circumstances.
Simplified: A vitamin is a chemical compound that is needed for the human body to work correctly. They include Vitamin A, many B vitamins (like B1, B2, B3, B6, and B12), Vitamin C, Vitamin D, Vitamin E, and Vitamin K. For example, citrus fruits such as oranges and lemons contain vitamin C.