A bachelor was a male individual who was eligible for, but not yet married. A bachelor getting married was sometimes celebrated with a bachelor party.
In an alternate timeline where Enterprise NX-01 from 2154 became stranded in 2037, several male Enterprise crewmembers, including Malcolm Reed, ended up as bachelors, since only one third of the ship's crew was female. (ENT: "E²")
In 2367, Geordi La Forge claimed that he enjoyed bachelor's life too much to enter a serious relationship. (TNG: "Identity Crisis")
In 2372, while Keiko O'Brien was on Bajor, her husband Miles O'Brien set up a workshop in their bedroom. When Keiko learned of this, she accused him of trying to live like a bachelor again. (DS9: "Hippocratic Oath") Later that year, when Keiko was about to return, Miles had to clean up their very untidy quarters, which Julian Bashir likened as a sculpture, calling it "a monument to your year as a bachelor." (DS9: "Accession")
In 2375, when the Silver Blood copy of Tom Paris was to be married to B'Elanna Torres, Kathryn Janeway told him that his bachelor days were over. (VOY: "Course: Oblivion")