Fraternizing refers to inefficient interaction between humanoids. (VOY: "The Gift" et. al.)
When the Enterprise was stranded in a neutronic storm, Captain Jonathan Archer described the situation to T'Pol as a chance to fraternize with the crew and get to know them. T'Pol replied that she was not good at fraternization, and Archer said it was a chance for her to learn. (ENT: "The Catwalk")
Lieutenant Malcolm Reed was trained not to fraternize with superior officers and felt that kind of socialization didn't have any place on a starship. (ENT: "Minefield") In 2152, while suffering the effects of radiation from a trinary star system, he expressed the opinion that he had tried to get Captain Jonathan Archer to play closer attention to security, "but he's more interested in fraternizing with the crew. Inviting them to breakfast, and to watch water polo." (ENT: "Singularity")
In 2371, Commander Chakotay suggested the possibility of a policy regarding fraternization amongst members of the USS Voyager crew when he observed two crewmembers kissing in a turbolift. (VOY: "Elogium")
Among the Quarren, efficiency monitors prevent employees from fraternizing. Seven of Nine, while brainwashed and serving as an efficiency monitor, prevented the also brainwashed Kathryn Janeway from doing so with Jaffen. (VOY: "Workforce")