Step 1: Understanding the Concept:
This question pertains to the philosophy of law, specifically the concept of "eternal law" within the natural law school of jurisprudence. Eternal law is seen as the highest form of law, derived from God's reason and will, governing the entire universe.
Step 2: Detailed Explanation:
The definition provided in the question is a classic formulation of eternal law from the writings of St. Augustine of Hippo (354–430 AD), one of the most influential Christian theologians and philosophers.
In his work, particularly in "Contra Faustum Manichaeum" (Against Faustus the Manichean), he defines eternal law (\textit{lex aeterna}) as "the reason or the will of God, who commands us to respect the natural order and forbids us to disturb it" (\textit{Lex aeterna est ratio divina vel voluntas Dei, ordinem naturalem conservari iubens, perturbari vetans}).
While other theologians and philosophers like St. Thomas Aquinas later developed the concept of eternal law further, this specific definition is famously attributed to St. Augustine.