A time machine was a device that allowed its operator to time travel.
This concept, if only presented as a work of fiction, dated back to the 19th century on Earth. The central theme of the novel The Time Machine, by H.G. Wells, was the titular device. When explaining to Michael Sullivan where she and the rest of USS Voyager's crew came from, "Katie O'Clare" asked him if he was familiar with the book. Michael took her question to mean that they had traveled backwards in time to Fair Haven, and she admitted that in a way, they had. (VOY: "Spirit Folk")
As a child, Malcolm Reed had always wanted to build a time machine to see the future. (ENT: "Future Tense")
When Jonathan Archer and Daniels became trapped in Earth's barren alternate 31st century, Archer, who recalled that Daniels said all of his time portals as well as all technology were gone, noted that there wasn't even any electricity there had any remained, asked if he needed to "find a bicycle, turn it into a time machine?" Daniels, who was attempting to be resourceful with what he had, replied, "Maybe we don't need a time machine," before taking Archer's communicator and scanner. (ENT: "Shockwave, Part II")
During Archer's confrontation with Vosk in an alternate 1944, Archer explained that he was aware that he and the other Na'kuhl were working with the Germans to get supplies and assistance in building a time machine to return to the 29th century. When the location of the device's construction was determined, Travis Mayweather overheard the Germans mention "something about a temporal conduit," adding, "I know it sounds crazy, but I think it's a time machine." (ENT: "Storm Front, Part II")