A Dear John letter was a type of letter written by an individual wishing to terminate a romantic relationship, addressed to the other party.
In 2374, Kathryn Janeway received a Dear John letter from her fiancé Mark Johnson. She said that she wasn't really surprised as she hadn't really expected him to wait for her given that Voyager was likely presumed lost. (VOY: "Hunters")
In 2376, upon realizing the ridiculousness of having a liaison with "Michael Sullivan", a hologram which she could alter to suit her taste in men, Janeway nearly wrote a letter to him to say goodbye. She found the idea of writing a Dear John letter to a hologram even more absurd. (VOY: "Fair Haven")
In 2377, Reginald Barclay visited Leosa's apartment and found it abandoned. He noted to Deanna Troi that there wasn't even so much as a "Dear Reg" letter left behind. (VOY: "Inside Man")