postpone vs reprieve

postpone is not a noun while reprieve is a noun.

Word	Noun	Adjective	Verb	Adverb
postpone	No	No	Yes	No
reprieve	Yes	No	Yes	No

As verbs, reprieve is a hyponym of postpone; that is, reprieve is a word with a more specific, narrower meaning than postpone:

postpone: hold back to a later time
reprieve: postpone the punishment of a convicted criminal, such as an execution

Other hyponyms of postpone include call, hold, suspend, probate, respite.

postpone (noun)	reprieve (noun)
	the act of reprieving; postponing or remitting punishment
	a warrant granting postponement (usually to postpone the execution of the death sentence)
	an interruption in the intensity or amount of something
	a (temporary) relief from harm or discomfort

postpone (verb)	reprieve (verb)
hold back to a later time	relieve temporarily
	postpone the punishment of a convicted criminal, such as an execution