Origins
Decree entered English in the 14th century from Old French 'decré,' from Latin 'dēcrētum' (an ordinance, a decree, a judicial decision), the neuter past participle of 'dēcernere' (to decide, to settle, to judge). The Latin verb compounds 'dē-' (down, thoroughly, completely) with 'cernere' (to sift, to separate, to decide, to perceive).
The PIE root *krey- (to sieve, to separate, to distinguish) gives 'cernere' a remarkably physical origin. The earliest sense was literal sifting — separating grain from chaff. From this arose the metaphor of mental sifting: distinguishing one thing from another, perceiving, judging, deciding. A 'dēcrētum' was something 'thoroughly sifted' — a matter fully considered and settled.
The 'cernere' family is surprisingly large. 'Discern' (from 'discernere,' to separate apart, to distinguish) preserves the sifting metaphor most directly. 'Certain' (from 'certus,' settled, sure, originally the past participle of 'cernere') describes something that has been decided beyond doubt. 'Concern' (from Medieval Latin 'concernere,' to sift together, to relate to) originally meant to mix or mingle, later shifting to mean involvement or worry. 'Crime' (from 'crīmen,' a charge, an accusation) comes from the same root via the sense of a judicial decision. Even 'secret' (from 'sēcernere,' to sift apart, to separate) and 'excrement' (from 'excernere,' to sift out) belong to this family.
Latin Roots
In Roman law, a 'dēcrētum' had specific technical meaning. It referred to a judicial decision by a magistrate, particularly a praetor or provincial governor, as opposed to an 'ēdictum' (a public proclamation) or a 'rescriptum' (a written reply to a legal question). Under the Empire, imperial decreta became a major source of law — the emperor's judicial rulings set binding precedent.
The word entered canon law through Gratian's 'Decretum' (c. 1140), formally titled 'Concordia discordantium canonum' (A Harmony of Conflicting Canons). This compilation of church law became the foundational text of Catholic canon law for centuries. Papal decrees — 'decretales' — were formal rulings on matters of doctrine and discipline, and they carried the force of law throughout Christendom.
In English law, a decree is an order of a court of equity (as opposed to a 'judgment' from a court of law). A 'decree nisi' (from Latin 'nisi,' unless) is a conditional decree that becomes absolute unless cause is shown — the term is particularly associated with divorce proceedings. A 'decree absolute' finalizes the order.
Modern Usage
Modern English uses 'decree' for any authoritative, unilateral order. Executive decrees, presidential decrees, and royal decrees all describe orders issued by a single authority without legislative deliberation. The word carries a faint whiff of absolutism — to govern 'by decree' implies ruling without consent or consultation.