The Colt .45 was a revolver popular in the 19th and 20th centuries. The .45 single action cavalry pistol was invented by Colt Firearms in 1873.
When a population of Humans were abducted from the American West by the Skagarans in the mid-19th century, they carried with them their technology, including the guns they used, such as the Colt .45. In the 2150s, when the Enterprise visited the Skagaran colony, the original guns or their re-creations were wielded by their descendants. (ENT: "North Star")
In 1893, Samuel Clemens owned a Colt .45 which was lost to time under San Francisco, only to be later discovered in the 24th century. (TNG: "Time's Arrow", "Time's Arrow, Part II")
In 2268, in the Melkotians reenactment of the Gunfight at the OK Corral, the entire landing party was given Colt .45s; however, they were never used, as they were given to them by people who were to execute them. The replicas of the Earp brothers also carried the same guns. (TOS: "Spectre of the Gun")
In 2369, in the holographic program Ancient West, the Colt .45 and its variants, the 'artillery' and the 'cavalry', was the standard revolver for the characters in this program. (TNG: "A Fistful of Datas")