the ".45" in .45 ACP refers to .45 inch diameter of the cartridge. the cartridge is the round or often called the "slug
", or "cap
". some of those terms are not 'technically' correct, but for the purpose of general slang they will do.
currently (circa 1999), the .45 ACP continues to rank among the great cartidges of all time, both as a target around and as ammunition for general shooting purposes.
the .45 Automatic Colt Pistol was designed by John M. Browning at some point prior to 1911.
*see also Colt .45 Automatic.