postcard vs post card

postcard and post card both are nouns.

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

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

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

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

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