'Decree' is Latin for 'a thing decided' — from 'cernere' (to sift). A ruling completely settled.
(noun) An official order issued by a legal authority, especially a head of state or government. (verb) To order something officially.
From Old French 'decré' (modern French 'décret'), from Latin 'dēcrētum' (an ordinance, a decree), the neuter past participle of 'dēcernere' (to decide, to settle, to decree), from 'dē-' (down, thoroughly) + 'cernere' (to sift, to separate, to decide). The PIE root is *krey- (to sieve, to separate, to distinguish). A decree was literally a matter that had been 'thoroughly sifted' — completely decided. The same root produced 'discern,' 'certain,' 'concern,' 'crime,' and 'crisis.' Key roots: dēcernere (Latin: "to decide, to settle
The Roman Senate's decisions were formally called 'senātūs cōnsulta' (resolutions of the Senate), but a 'dēcrētum' originally referred to a specific judicial or magisterial ruling. When Gratian compiled his enormously influential collection of canon law around 1140, he titled it 'Concordia discordantium canonum' — but everyone just called it 'the Decretum,' cementing the word in European legal vocabulary.