Junk mail and Post card

Junk mail noun - Third-class mail consisting of advertising and often addressed to `resident' or `occupant'.

Post card and junk mail are semantically related In some cases you can replace phrase "Post card" with "Junk mail", this phrases are similar.

Post card noun - A card for sending messages by post without an envelope.

Junk mail and post card are semantically related in written communication topic. Sometimes you can use "Junk mail" instead a noun phrase "Post card", if it concerns topics such as written correspondence.