In tourism and travel industry, PAX means passenger, travelers, or occupants.
We can define PAX as travelers (live human bodies that need to be transported) generally applied to any "customers, people, bodies" occupying physical space (usually, seats or beds) and/or services, once they often must be entertained, accommodated, fed, etc.
It's a very common word, used broadly by professionals of the industry of Travel and Tourism.
PAX term can be used in all kind of tourism products, as flights, hotel, cars, boats, tickets, cruises, trains, shows, etc. A PNR can cointain PAX information. According to the New Oxford American Dictionary, the plural of PAX is still PAX.
On the magazine “Air facts, Incorporated” Volume 9, page 24, dating back from 1946, the term PAX was already used referring as a passenger.
GDS systems like Amadeus, Sabre and Travelport, also uses the acronym PAX to reference passengers.