Any compound capable of being converted into a vitamin by the body.
Source: Fintess-Web.com Vitamins
glossary
Beta-carotene (found in orange and yellow fruits and vegetables) is an important antioxidant and an example of provitamin A because it can be converted to Vitamin A by the human body.
Source: GreenFacts
GreenFacts Summary on Genetically Modified Crops:
Español: Provitamina
Français: Provitamine