A souvenir, memento, trinket, knickknack or tchotchke was an item meant to memorialize a past experience. They were often sold at gift shops.
Trip Tucker promised to bring T'Pol a souvenir from his shore leave on Risa in 2151. (ENT: "Two Days and Two Nights")
When Enterprise NX-01 was infested by a symbiotic lifeform in 2152, Malcolm Reed wondered if it could have been a souvenir left by the Kreetassans. (ENT: "Vox Sola")
In 2153, after Trip Tucker noted that Phlox would have loved the Xanthan floating bazaar, Jonathan Archer said "souvenir hunting" would have to wait. (ENT: "Rajiin")
Later the same year, Trip Tucker asked Malcolm Reed to bring him back a piece of the Xindi weapon as a souvenir. (ENT: "Countdown")
In 2268, Elaan gave James T. Kirk her dagger as a personal memento. (TOS: "Elaan of Troyius")
In 2366, Geordi La Forge called six explosive devices collected from the Koinonian homeworld souvenirs. (TNG: "The Bonding")
Later that year, William T. Riker asked Jean-Luc Picard to bring him a horga'hn from Risa as a souvenir. (TNG: "Captain's Holiday")
Guinan called a powerful rifle she used to restore order to Ten Forward in 2367 "a little souvenir [she] picked up on Magus III". (TNG: "Night Terrors")
In 2369, Quark explained to Odo his vision of expanding into family entertainment, including "Ferengis in every doorway selling useless souvenirs". (DS9: "If Wishes Were Horses")
While evacuating Deep Space 9 in 2370, Quark told Julian Bashir that the large case he carried contained personal mementos. Bashir correctly surmised that it was actually full of gold-pressed latinum. (DS9: "The Siege")
Also in 2370, Jean-Luc Picard referred to a mask of Masaka created by Data as a memento of the latter's experience with the D'Arsay archive. (TNG: "Masks"
In early 2371, Quark tried to sell Harry Kim a lobi crystal as "a memento of [his] first mission" for his parents. (VOY: "Caretaker")
Later that year, Chakotay told B'Elanna Torres about how he once unknowingly desecrated a Ktarian grave on Ktaria VII by removing a stone as a memento. (VOY: "Emanations")
In late 2371, Miles O'Brien tossed Michael Eddington a tetryon compositor the latter had sabotaged, calling it a souvenir. (DS9: "The Die is Cast")
In an alternate 2374 where USS Voyager engaged in a prolonged conflict with the Krenim, Kathryn Janeway called some burn scars she gained from repairing the main deflector mementos. (VOY: "Year of Hell, Part II")
In 2375, Odo identified an object he had purchased while at a conference as a knickknack. According to the Changeling, the shopkeeper he had bought it from referred to it thusly. Miles O'Brien was surprised to learn that Odo was interested in such things, but Odo clarified that he had purchased the object for Kira Nerys. (DS9: "Chimera")
During a near-death hallucination experienced by B'Elanna Torres in 2376, Chakotay called a Klingon artifact that damaged her shuttle a souvenir. (VOY: "Barge of the Dead")
Later that year, Rudy Ransom claimed to Kathryn Janeway that he wanted to collect a few mementos from his quarters on the USS Equinox, when in fact he was plotting to leave USS Voyager behind. (VOY: "Equinox")
In 2377, Kathryn Janeway told Jaffen that she would not need souvenirs to remember him. (VOY: "Workforce, Part II")
In 2378, Neelix suggested Seven of Nine could make her quarters on USS Voyager more homey by adding decorations, such as "photographs, artwork, knick-knacks". Seven was confused by the term, and the Talaxian explained, "Little things, like the wood carvings in Ensign Kim's quarters." Seven had never seen the items he spoke of, but conceded that she could replicate some items. (VOY: "Human Error")
Later that year, Tom Paris picked up a music-playing toy on Otrin's homeworld as a souvenir. (VOY: "Friendship One")
In 2381, Trade Minister Korzak of the Karemma dismissed the shops on Deep Space 9's Promenade as "tchotchke kiosks". (LD: "Hear All, Trust Nothing")