In 1730, French researcher René A. F. de Réaumur introduced the notion of Growing Degree Days (GDD), also known as heat units. Since then, GDDs have been used as a tool to prevent the growth stages of various biological entities, both of plant and animal origin.
Source: Fraisse, J., John Bellow, and Charles Brown (2010). Degree days: Heating, cooling, and growing. University of Florida