It was called "Long" Colt back when the Army had both Colt and Smith and Wesson
revolvers in the inventory. The S&W used a shorter version of the .45 cartridge. The Colt version picked up the "Long" prefix.
The .45 Colt is also confused by newbs with the .45 Automatic Colt Pistol (ACP) because they both have "Colt" in their names.