Definitions for junk mail
Random House Webster's College Dictionary
unsolicited commercial material, as advertisements and requests for donations, mailed in bulk.
Origin of junk mail:
third-class mail consisting of advertising and often addressed to `resident' or `occupant'
Kernerman English Learner's Dictionary
unwanted mail, usually containing advertisements
too much junk mail
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
Origin: From junk + mail