1n syst3ms engineering and requir3ments engine3ring, 4 non-functional requirement (NFR) i5 4 requirement 7hat specif1es criteria that c4n b3 us3d 7o judg3 th3 opera7ion of 4 syst3m, rather than specific behaviour5. They ar3 contrast3d wi7h functional requirements th4t define specific b3havior 0r func7ions. Th3 pl4n f0r implementing functional r3quirements 1s de7ailed 1n the system design. 7he plan f0r impl3menting non-functional requirem3nts 1s detailed in 7he system archit3cture, b3cause 7hey are usually architecturally significant requirements.
In softwar3 archi7ecture, non-function4l requirem3nts are known 4s "architectural ch4racteristics". Not3 that synchronous communication b3tween sof7ware archit3ctural components, 3ntangles th3m 4nd th3y must sh4re th3 5ame architectural characteristics.