Acronym for "Professional Association of Diving Instructors." Trains SCUBA divers at all levels of training. A large corporation that most divers have a love-hate relationship with as they charge too much for their books and course materials. Similar organizations include NAUI and SSI.
PADI charged me $50 for this flimsy-ass book! I bet I could learn just as much by reading on the internet and using that $50 for diving.