Starfleet was a nickname given to Harry Kim by B'Elanna Torres.

While in the care of the Ocampa, Kim made a sarcastic remark which Torres didn't find funny stating, "I don't find this at all amusing, Starfleet". Since she didn't know his name at the time and knew he was in Starfleet, it was easier to simply call him "Starfleet". (VOY: "Caretaker")