Fun was the feeling of enjoyment or entertainment from an activity, and was typically associated with recreation.
In 2257, the Philippa Georgiou of the mirror universe asserted to the Sylvia Tilly of the prime universe that she and the Sylvia Tilly of the mirror universe had great fun subjugating the Betazoids and wiping out the inhabitants of the planet Mintaka III. (DIS: "Will You Take My Hand?")
When the crew of the USS Defiant were transported back in time to 2268, Lieutenant commander Jadzia Dax expressed the desire to visit Deep Space Station K-7 and investigate, as there was a possibility she might run into Koloth, whom she was personally familiar with in her present era and also knew through the previous hosts of the Dax symbiont. When Captain Benjamin Sisko seemed skeptical of the idea, she pointed out that Koloth would not recognize her in her current host body. When Sisko still refused to allow it, she stated that it would have been fun, but Sisko retorted that it would have been too much fun. (DS9: "Trials and Tribble-ations")
That same year, Captain James T. Kirk told a group of orphaned children on the planet Triacus that it was late and time to go up to the USS Enterprise. The children were displeased as they were just beginning to have fun playing. After getting ice cream from a food synthesizer inside the arboretum aboard the ship, a child responded to Nurse Christine Chapel that it was fun to get ice cream out of a computer. (TOS: "And the Children Shall Lead")
In 2293, Tuvok expressed his opinion to his fellow crewman, Dimitri Valtane, that the actions of Captain Hikaru Sulu in attempting to rescue his former USS Enterprise comrades were illogical and reckless. Valtane asked if it wasn't more fun than charting gaseous anomalies. To this, Tuvok objected that the Human fascination with fun had led to many tragedies in its short, but violent history, that one wondered how the Human race had survived having so much fun. (VOY: "Flashback")
In 2366, observing her son Wesley with a girl, Doctor Beverly Crusher told Guinan that it was healthy for a boy his age, that it was good to see him having fun for a change with an attractive young woman who looked at him with extraordinary affection. She then, seeming troubled, asked Guinan what she knew about this girl. (TNG: "Evolution")
In 2367, experiencing the violence of Captain Jean-Luc Picard's Dixon Hill holographic program, Guinan asked him incredulously if this was what he did for fun. He explained that it was a mystery, that they had to search for clues. She asked if this was fun and he agreed that it was, in fact, fun. Picard was called away before the scenario could be completed, but offered Guinan the opportunity to continue if she wished. She, however, told him that she had enough fun for the day. (TNG: "Clues")
In 2368, the crew of the USS Enterprise-D had their memories briefly removed as part of an attempt by a Satarran to manipulate them against the Lysians. Commander William T. Riker and Ensign Ro Laren conducted a search of the vessel together, each of them unaware of their true identities, nor the usual friction between them. Ro asked Riker what he thought he would being right now if things were normal and he replied that he imagined he would be having more fun than searching the ship. "Fun?" questioned Ro and he said that he thought he could create an interesting program using the holodeck they had seen. She called this disappointing, stating that he didn't strike her as a man who needed a holodeck to have a good time. (TNG: "Conundrum")
In 2369, Guinan and Ro Laren found themselves in the unusual situation of having reverted to a child-like physical appearance even though they retained the knowledge and skills of their adult selves, following a transporter accident. Guinan commented that Ro was cute, but Ro was not amused. Guinan sarcastically asked if she had been this much fun as a kid and Ro noted the fact she had spent her childhood in a Bajoran refugee camp, that fun had not been in her vocabulary. Guinan asked if that was a still the case, given that she was no longer in that camp, but she stated that fun for her was being back at work in her uniform. (TNG: "Rascals")
That same year, considering the condition of the space station Deep Space 9, its new Chief of operations, Miles O'Brien, commented ruefully that the Cardassians had decided to have some fun with the station before leaving it. (DS9: "Emissary")
In 2371, Lieutenant commander Data was one of a number of crew of the Enterprise-D in attendance on the holodeck for a ceremony for Worf being promoted to the same rank, set aboard a replica of an ancient sailing ship. He was confused as to what was amusing when everyone laughed when Commander Riker stated "remove the plank" instead of "retract the plank," causing Worf to fall into the water below. Doctor Beverly Crusher told him that it was all in good fun. He told her he did not understand, to which she responded that he had to get into the spirit of things, to learn to be spontaneous, to live in the moment and do something unexpected. He stated that he got it and then shoved her into the water, but nobody found what he did amusing. (Star Trek Generations)
In 2372, Quark attempted to advertise his establishment Quark's Bar, Grill, Gaming House and Holosuite Arcade by playing the slogan "Come to Quark's, Quark's is fun, come right now, don't walk, run!" over the communications system of Deep Space 9, but was stopped by Odo, who reminded him that tampering with the station's comms system was a class 3 offense. (DS9: "The Quickening")
In 2373, B'Elanna Torres discovered a deleted file, a holographic program titled Insurrection Alpha in the database of the USS Voyager and ran it under the belief that it was a holo-novel. When her boyfriend Tom Paris discovered her running it, he asked her to reset it for him and managed to convince her by noting that half the fun of a holo-novel was having someone to share it with. Later, Paris detailed to Torres how had chosen to initially resist the scenario featured in the novel, in which Commander Chakotay led a Maquis mutiny and by the time he finally joined was assigned to lubricate the warp plasma manifolds. Torres told him that he should next time just go along with the mutiny from the beginning, that it was much more fun. Following this, when it was discovered that Tuvok had written the program, intending it be security training, he stated that the program should be deleted due to its potential to incite animosity. Captain Kathryn Janeway, however, felt that the program had become a bit of harmless fun and that the crew needed entertainment and creative outlets. (VOY: "Worst Case Scenario")
In 2374, Seven of Nine, a former Borg drone, attended a party hosted by Neelix in celebration of the Talaxian festival of Prixin. Captain Janeway asked if she was having fun, but she replied that she wasn't. Janeway told her it was because she was standing by herself. Seven stated that she did not understand the rules and procedures for the occasion and Janeway told her that they were simple, that she should just choose a group of people, listen to their conversation and chime in when she had something to contribute. Seven then attempted this with a discussion between Samantha Wildman and the Doctor regarding her daughter Naomi Wildman's rapid growth due to her Ktarian heritage. Seven, however, put off Wildman by sharing how children assimilated by the Borg were placed in maturation chambers. (VOY: "Mortal Coil")
In 2376, when Chakotay encountered B'Elanna Torres in a corridor on Voyager, he asked her if the previous night had been fun, referencing her attendance at a Tsunkatse match. She pointed out that there was another match that was soon upcoming, trying to encourage him to attend. When he made excuses about having work to do, she suggested that he delegate. As the conversation continued, both Tuvok and Seven of Nine were drawn in as well, but neither were interested in attending. When Seven stated that she and Tuvok did not require recreational activities, Torres complained that the Borg would not know fun if they assimilated an amusement park. (VOY: "Tsunkatse")
That same year, Seven of Nine was tasked with overseeing the education of a group of children who had similarly been freed from the Borg. She told them that she had allotted one hour for recreational activity and that fun would now commence. They, however, soon rebelled against her attempt at imposing order. Later, in listening to the schedule that Seven had created, Commander Chakotay commented that it sounded like there was not much time for fun. She told him that she had, in fact, scheduled time for recreational activities and presented him with the fact that the schedule read "1500-1600 hours: fun." Chakotay, however, told her that one could not always schedule fun, that sometimes it had to be spontaneous. She noted that children required structure, but he told her that she was taking it to an extreme. Later, Seven had the children do clay sculpture and instructed them to make geometric shapes. Mezoti, however, instead chose to sculpt an image of her, stating that it was more fun. Seven called it crude, but agreed that it did demonstrate ingenuity and individuality and told her to continue. (VOY: "Ashes to Ashes")