A provitamin is a substance that may be converted within the body to a vitamin.
The term "previtamin" is a synonym.
For example, Provitamin B5 is a name for Panthenol

Panthenol

Panthenol is the alcohol analog of pantothenic acid , and is thus a provitamin of B5. In organisms it is quickly oxidized to pantothenate. Panthenol is a highly viscous transparent liquid at room temperature, but salts of pantothenic acid are powders...

, which may be converted in the body to Pantothenic Acid

Pantothenic acid

Pantothenic acid, also called pantothenate or vitamin B5 , is a water-soluble vitamin. For many animals, pantothenic acid is an essential nutrient. Animals require pantothenic acid to synthesize coenzyme-A , as well as to synthesize and metabolize proteins, carbohydrates, and fats.Pantothenic acid...

 (Vitamin B5).

The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converted to an active form by normal metabolic processes. For example, "Provitamin A" is a name for Beta-Carotene, which has only about 1/6 the biological activity of Retinol (Vitamin A) -- the body converts Beta-Carotene to Retinol. In other contexts, both Beta-Carotene and Retinol are simply considered to be different forms (vitamer

Vitamer

A vitamer of a particular vitamin is any of a number of chemical substances, each of which shows vitamin activity. Very commonly each "vitamin" is not a single chemical, but rather multiple chemical substances called vitamers, each of which is defined by its different biological activity.For...

s) of Vitamin A.