reprieve vs postpone

reprieve is a noun but postpone is not a noun.

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

As verbs, postpone is a hypernym of reprieve; that is, postpone is a word with a broader meaning than reprieve:

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

Other hypernyms of reprieve include defer, hold over, prorogue, put off, put over, remit, set back, shelve, table.

reprieve (noun)	postpone (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

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