A letter (or e-mail in today's world) that is used by a woman to break up with their boyfriend from a distance. Usually used by girls who have boyfriends in the military deployed for extended periods of time.

Susie couldn't wait for her boyfriend Sam to finish his one year deployment in Iraq, so she sent him a Dear John letter over the Internet three months into his deployment to tell him she was seeing someone else.

A "Dear John letter" is a letter written to a husband or boyfriend to inform him their relationship is over, usually because the author has found another lover. Dear John Letters are often written out of an inability or unwillingness to inform the man in person. The reverse situation, in which someone writes to his wife or girlfriend to break off the relationship, is referred to as a "Dear Jane letter".

"Dear John..
I'm writing to inform you that I've found another replacement to my lonely vagina, please understand that, I'm a whore and needed another man.
With Love,

A break up letter sent to a girl's significant other. Generally used in military relationships, while the man is deployed for a long period of time. Usually, Dear John letters are reserved for bitches who can't be proud of their man who is risking his life to protect his country.

She started sleeping around with other guys while her fiance was deployed to Afghanistan, so she wrote him a Dear John letter to break off the engagement.