There are lots of different .45 loads. In rough order of development:
1. .45 - sometimes called ".45 Colt", although this is historically inaccurate. This was the load developed for the Colt Model P revolver (known to history as the Single Action Army, or SAA, or colloquially as the "Peacemaker) in 1873.
2. .45 Schofield, or ".45 Short" - developed for the Smith & Wesson Schofield revolver. It was shorter than the original .45, which was too long to fit the Schofield's cylinder. The US Government standardised on this round for a time in the late 19th century, as it was interchangeable between the SAA and Schofield.
3. .45-70 Government - a rifle round, developed during the 1870's for US service rifles. Used by Custer's men at the Little Big Horn, and still popular today for bear defence.
4. .455 - a British round, used in Webley revolvers.
5. .45 ACP - early 1900's, developed without a rim for use in Colt pistols, hence the initials after the number ("Automatic Colt Pistol").
6. .45 GAP - developed in the 1990's for Glock, a shorter, higher-pressure version of the .45 ACP.
There are probably a few others floating around.