Fraternization refers to non-professional interactions between individuals in the same organization.
When Enterprise NX-01 was trapped in a neutronic storm, Captain Jonathan Archer described the situation to Sub-Commander 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. T'Pol would later take Archer's suggestion, joining the crew for a presentation of their Tuesday weekly movie night. (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")
Michael Burnham claimed that her position of first officer aboard the USS Shenzhou had prevented her from partying with the crew, since it would have been inappropriate fraternization. (DIS: "Magic to Make the Sanest Man Go Mad")
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")