The most recent edition of Webster’s New Universal Unabridged Dictionary defines junk mail as:

junk mail:  
unsolicited commercial mail (1950-55).

junk mailer:  
1.  an organization that sends junk mail in bulk, esp. to solicit business or charitable contributions;
2.  a business that specializes in preparing and distributing junk mail for others.

Junk mail, sometimes termed “direct” or “bulk” mail, consists of hundreds of billions of pieces of written postal mail sent annually to unsuspecting people which advertise a product or service and solicits orders. Junk mail is officially called “standard mail” by the United States Postal Service (USPS). Customarily, this mail is completely unlike normal letters—it is usually not requested or sought by you, but results from a distribution of details about your personal identity and location for marketing and sales uses. From city high rises to farms to suburban neighborhoods to beach homes, the uninvited guest known as junk mail arrives at doorsteps across the country. Junk mail is the creature of large direct mail companies who enter into reduced postage rate arrangements with the USPS in order to send massive amounts of paper catalogues, solicitations, coupons, postcards and flyers to mailing addresses across the nation. Regretfully, neither the U.S.Postal Service nor your friendly postal carrier can stop junk mail because as long as mail is lawful and with proper postage, it has to be delivered.