A letter from one's fiancée or girlfriend breaking off an engagement or love affair, or from one's wife asking for a divorce.
Origin of dear-john-letter
While the exact origins of the phrase are unknown, it is commonly believed to have been invented by Americans during World War II. Large numbers of American troops were stationed overseas for many months or years, and as time passed many of their wives or girlfriends decided to begin a relationship with a new man rather than wait for their old one to return. As letters to servicemen from wives or girlfriends back home would typically contain affectionate language, a serviceman receiving a note beginning with a curt "Dear John" (as opposed to the expected "Dear Johnny", "My dearest John" or simply "Darling" for example) would instantly be aware of the letter's purpose.