We can start with the statement that a vitamin is an organic compound required in tiny amounts by an organism for its metabolism.
So, are things like amino acids, fatty acids or carbohydrates vitamins? No - they are required in large amounts (not tiny amounts).
And what about substances like magnesium, zinc and the like? Well, they are required in tiny amounts, but they aren't organic compounds (they're 'inorganic' elements).
But there are still lots of rare organic compounds that they body makes that don't qualify as vitamins. Why not? Well, there is one further criterion to include - vitamins can't (by definition) be synthesised in sufficient quantities by the body. A few of them can be synthesised a little, but none in amounts adequate to our need. Rather, vitamins need to be ingested in our diets; we take them from the plants and lower animals that can produce them.
So, in conclusion:
- organic compounds,
- required in tiny amounts
by an organism for its metabolism. By definition, they
- can't be synthesised in adequate amounts by the organism, and must thus be ingested.
Works for me...