n. A rectangular piece of thick paper or thin cardboard intended to be written on and mailed without an envelope. In the case of a picture postcard one side carries a picture or photograph.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. a card for sending messages by post without an envelope
Word Usage
"This postcard is a little thicker than average, in order to create the effect, so naturally the Postal Service did their best to destroy it."