What Is a Vitamin? Towards a Contemporary Definition

<b>Background:</b> The concept of vitamins has evolved over the past century from compounds preventing classical deficiency diseases to nutrients recognized for supporting long-term health. Despite their central role in science and public health, existing definitions often fail to clearly characterize and distinguish vitamins from other bioactive compounds and do not capture the complexity of their nutritional requirements. <b>Method:</b> This article reviews the historical origins and current definitions of vitamins. <b>Results:</b> We identify the limitations of existing definitions and present a contemporary, physiologically informed definition as a discussion proposal. Our proposal no longer relies solely on the prevention of classical hypo- or avitaminoses. <b>Conclusions:</b> By incorporating the concept of conditional essentiality, this framework also tries to clarify the distinction between classical vitamins and other bioactive substances, reflecting variable dietary requirements under different conditions.