Junk mail and Third class

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

Third class and junk mail are semantically related. In some cases you can use "Third class" instead a noun phrase "Junk mail".

Third class noun - Inexpensive accommodations on a ship or train.

Junk mail and third class are semantically related. Sometimes you can replace phrase "Junk mail" with "Third class", this phrases are similar.