A revenue passenger is someone who has paid the transport operator for her or his trip. That excludes non-paying passengers such as airline employees flying on free or nearly-free passes, babies and children who do not have a seat of their own, etc. However, passengers who paid for their trip with a frequent-flyer program mileage award are usually included.
This term is used in the transportation industry, in particular in traffic measures such as RPK (revenue passenger kilometer) or RPM (revenue passenger mile).