Origin and Destination passengers.
This is the data that is used by airports and airlines alike (as supplied to and tabulated by the USDOT) to calculate existing demand between 2 airports/markets. So if I were to tell you that the O&D between PVD and LAX
is 300 - that simply means, on average, there are 150 people per day, each way, traveling between those 2 places regardless of where they stopped, connected, or how they got there as long as it was on commercial air service.
Some people on here will try to tell you that this info isn't all that accurate, but I can assure you, this is the data that the airline planning departments use/start with when exploring route opportunities. O&D doesn't necessary tell you what the potential demand for a market is - just what was actually there for a given quarter. Lots of other things effect O&D (fares, nonstop service, frequency, competition, etc., etc.).