from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Third-class mail, such as advertisements, mailed indiscriminately in large quantities.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. any mail or letters that are not welcome or solicited and typically sent in bulk; especially mail of a commercial nature such as advertising circulars and form letters
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. third-class mail consisting of advertising and often addressed to `resident' or `occupant'
From junk + mail (Wiktionary)