Ordain

v. empower to function in a religious office; determine law, establish order; destine, set apart for a particular purpose or function; command

Holy Orders in the Roman Catholic (Latin: sacri ordines), Eastern Catholic, Eastern Orthodox (ιερωσυνη, ιερατευμα, Свештенство), Oriental Orthodox, Anglican, Assyrian, Old Catholic, Independent Catholic churches and some Lutheran churches are the three orders of bishop, priest and deacon, or the sacrament or rite by which candidates are ordained to those orders.
See more at Wikipedia.org...

Verb
1. order by virtue of superior authority; decree; "The King ordained the persecution and expulsion of the Jews"; "the legislature enacted this law in 1985"
(synonym) enact
(hypernym) decree
(hyponym) reenact
2. appoint to a clerical posts; "he was ordained in the Church"
(synonym) consecrate, ordinate, order
(hypernym) invest, vest, enthrone
(derivation) ordainer
3. invest with ministerial or priestly authority; "The minister was ordained only last month"
(hypernym) invest, vest, enthrone
(derivation) ordainer
4. issue an order
(hypernym) decree
(hyponym) predestine
(derivation) ordinance

(v. t.)
To set in order; to arrange according to rule; to regulate; to set; to establish.
  

(v. t.)
To set apart for an office; to appoint.
  

(v. t.)
To regulate, or establish, by appointment, decree, or law; to constitute; to decree; to appoint; to institute.
  

(v. t.)
To invest with ministerial or sacerdotal functions; to introduce into the office of the Christian ministry, by the laying on of hands, or other forms; to set apart by the ceremony of ordination.
  

To ordain is to make an ordinance, to enact a law.

In the Constitution of the United States, the preamble declares that the people "do ordain and establish this constitution for the United States of America." The 3rd article of the same Constitution declares, that "the judicial power shall be vested in one supreme court, and in such inferior courts as the congress may from time to time ordain and establish."
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.