The story
From America's Cup + 45, because it measures approximately 45 feet on the waterline.
Etymology from Wiktionary, CC BY-SA 4.0.
AC45
Today it means: A catamaran sailing racing yacht, with a wingsail mainsail, developed for the 2013 America's Cup world tour, the 2013 America's Cup pre-regatta tune-up league, called the America's Cup World Series. Full definition →