Requirement (-ment), n.
1. The act of requiring; demand; requisition.
2. That which is required; an imperative or authoritative command; an essential condition; something needed or necessary; a need. One of those who believe that they can fill up every requirement contained in the rule of righteousness. J. M. Mason. God gave her the child, and gave her too an instinctive knowledge of its nature and requirements. Hawthorne.
Shades of meaning
Fernald has no entry of its own for requirement; he lists it under necessity, order.
Where he writes about it:
Command is a loftier word, as well as highly authoritative, less frequent in common life; we speak of the commands of God, or sometimes, by polite hyperbole, ask of a friend, "Have you any commands for me?" A requirement is imperative, but not always formal, nor made by a personal agent; it may be in the nature of things; as, the requirements of the position. — from order
Prohibition is wholly negative; it is a command not to do; injunction is now oftenest so used, especially as the requirement by legal authority that certain action be suspended or refrained from, pending final legal decision. — from order