post card vs postcard

post card and postcard both are nouns.

Word	Noun	Adjective	Verb	Adverb
post card	Yes	No	No	No
postcard	Yes	No	No	No

As nouns, post card and postcard are synonyms defined as:

post card and postcard: a card for sending messages by post without an envelope

Other synonyms of post card include mailing-card, postal card.

post card (noun)	postcard (noun)
a card for sending messages by post without an envelope	a card for sending messages by post without an envelope