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
Junk mail is advertisements and publicity materials that you receive through the post which you have not asked for and which you do not want. Third-class mail, such as advertisements, mailed indiscriminately in large quantities. letters, especially advertisements, that are sent by organizations to large numbers of people - used to show disapproval