What do the letters "ACP" in "45ACP" stand for?

ACP stands for automatic Colt pistol. The term is associated with several types of ammunition developed by prolific American firearms designer John Browning for the Colt Company. ACP ammo is commonly used in semi-automatic pistols.

The .45 is the most commonly used weapon in the ACP family. Adopted by the U.S. military in 1911, it was chosen following the disappointing performance of the smaller .38 caliber pistols used during the 1904 American conflict in the Philippines. The .45 offers excellent stopping power and superb reliability. In 2011, the ammo celebrated a century of use thanks to its reputation for dependability.