More of a slang term, not a misnomer.
The .45 Colt, sometimes called the .45 Long Colt to distinguish it from the short .45 cartridge historically required by the S&W Schofield revolver, as well as the .45 ACP (also sometimes, incorrectly, referred to as the .45 Colt), is the oldest service cartridge still in use today. It was introduced in 1873 for the then new Colt Single Action Army (SAA) revolver, the famous Peacemaker. The .45 Colt and the SAA revolver quickly became legends in their own time, and the most popular handgun/cartridge combination on the Western frontier.