Award tickets: airline tickets paid for using miles.
Revenue tickets: airline tickets paid for using money.
Redeemable miles: miles earned that can be redeemed for award tickets (though miles can occasionally be used for other things, such as TSA Pre-check, in the case of United, and drinks in lounge, in the case of Delta).
Elite qualifying miles: miles earned that count towards elite status, but cannot be redeemed.
Elite qualifying dollars: dollars spent for travel on a certain airline that count towards elite status. In some programs, one can achieve elite status by spending a certain amount of money (this is unique to airlines that have revenue-based programs).
F/J/W/Y: One-letter codes used to refer to airline classes of service. F corresponds to first, J to business, W to premium economy, and Y to economy. Not to be confused with fare classes.
Fare classes (F/A/I/J/P/Y/B/K/M and many others): one-letter codes airline use to refer to different types of fares. There are more out there than can be counted, and the same codes often mean different things on different airlines.
Wide-body aircraft: An aircraft with two aisles (current examples: the A380, A350, A340, A330, 767, 777, 787, and 747)
Narrow-body aircraft: An aircraft with one aisle (current examples: the 737, A320, and, technically, every regional jet every made)
Twin engine: an aircraft with two engines.
Regional jet: a small aircraft, typically with less than 100 seats. With a few exceptions, most regional jet flights are short and link hubs with smaller cities.
Mainline: flights operated by an airline itself instead of by its regional subsidiary or regional code-share partners.
US 3: A term used to refer to the three major US carriers, Delta, American, and United. With Alaska Airlines becoming a increasingly big player in the industry, you’ll sometimes see this written as US 4.
ME 3: An abbreviation to ‘Middle East 3,’ a term used to refer to the 3 biggest carriers in the Middle East, Etihad, Emirates, and Qatar.