QSI– Quality of Service Index 
An analytical process of allocating percentage shares of demand on a particular O&D to the various possible itineraries (consisting of particular carriers, flight numbers, times of day, equipment types, days of the week, etc.) by determining, from the flight schedule, all eligible routes between the O & D, scoring them for service quality (with preferences for things like fewer connections, better routings, faster planes, etc.), and determining a percentage distribution based on score. 
For example, give each airline QSI points for each direct, single, and double connect trip they operate. Direct flights are the most desirable, so they get the most points. Figure out the percentage of QSI each airline holds for a certain route. The equipment also factors into the number of points allocated. 

circuity how direct your single- or multi-connect is; i.e. how far out of the way it ends up going. For example, a direct flight from IAD-LAX is 2,288 miles. However, IAD-LAX connecting in ATL is 533 miles (IAD-ATL) + 1,946 miles (ATL-LAX), for a total of 2,479 miles. The circuity factor is 2,479 / 2,288 = 1.083.

great-circle miles – the direct distance between two airports, calculated from their latitudes and longitudes and trigonometric functions related to the curvature of the earth. There are at least two formulas used to calculate GCD (great circle distance) miles. The simple formula wrongly assumes the earth is a perfect sphere but is faster and easier to use for general calculations; the substantially more complicated formula accounts for the fact that the earth is slightly pear-shaped. 

schedule– the “Schedule” is the master listing of all scheduled flights, including origin, destination, date, time, equipment type, marketing airline, operating airline, via points, etc. Schedules are usually updated weekly. There are two primary schedule vendors, OAG (Official Airline Guide), and Cirium.