There seems to be some confusion surrounding terms the airline industry uses. For instance, many people believe “nonstop” and “direct” flights are the same thing. They are not. Neither are a “connection” and “stopover.” And then there are all those baffling acronyms. NDC. LLA. LLC. FBC.

Whether you’re helping travelers with a complicated flight itinerary, seeking cost-savings solutions to optimize your company’s air program or simply wish to brush up on some aviation jargon, here’s a glossary to translate some terms often used in the industry.

Air Re-Shop Expert™: an airfare re-shopping solution from American Express Global Business Travel (GBT) that automatically rebooks/reissues a ticket if a lower price is found within 24 hours of booking

Air Track Expert™: an automated tool from GBT that identifies and redeems/repurposes unused e-tickets from within the past 12 months with options available for refundable and nonrefundable tickets

ancillary fees: aka “optional services”; additional amenities airlines offer, such as seat allocation, fast-track boarding, onboard meals, etc., that may be bundled in the product offer or offered as additional à la carte services

blackout period: specific dates when the special rates you’ve negotiated with a preferred airline supplier do not apply due to high demand

branded fare: when an airline bundles an airfare with ancillary services, such as priority boarding, checked luggage and an in-flight meal

bumped: when a passenger is denied a seat on an aircraft due to the flight being oversold; in such a situation, the passenger is entitled to compensation if the airline cannot make alternate arrangements that get them to their destination within one hour of the original arrival time

cabin class: not to be confused with “fare class”; a section of an aircraft in which passengers travel; there are four levels of cabin classes: economy, premium economy, business and first

capacity-controlled fare: when an airfare increases or decreases depending on the number of passengers and seat allocation/supply

connection: aka “layover,” not to be confused with “stopover”; when a traveler changes planes to connect to another flight to reach their destination and the stop is less than four hours domestically or 24 hours internationally; “online connections” involve a change of aircraft but not airline, whereas “offline connections” involve both

direct flight: not to be confused with a “nonstop flight”; a flight where the passenger does not need to change planes, but the aircraft may stop in one or more cities to reach the destination

dynamic pricing: methodology used in the airline industry that adjusts fares based on factors like remaining capacity, average demand for certain routes and the probability of selling more seats later; some airlines also recently have introduced software that sets fares according to a traveler’s personal flight history

fare class: aka “booking class,” not to be confused with “cabin class”; designations that define the cabin class and benefits/rules associated with a certain airfare (such as refundable vs. nonrefundable, number of rewards miles earned, etc.)

FBC: fare basis code; an alphabetical/numeric code that airlines use to define the rules associated with different types of airfares

GDS: global distribution system; a worldwide reservation network used as a single point of access for reserving airline seats, hotel rooms, rental cars and other travel-related items by travel agents, online reservation sites and large corporations

International Rate Desk Services: centralized, specialized services offered by GBT for multi-leg, international, complex itineraries to maximize savings

LLA: lowest logical airfare; the lowest airfare a business traveler can book that is consistent with guidelines of a company’s travel policy

LCC: low-cost carrier, aka “no-frills airlines”; these carriers offer low-fare flights with reduced passenger services; EasyJet, Ryanair, AirAsia and Southwest are some examples

NDC: New Distribution Capability; a set of technology standards intended to improve the airlines’ ability to sell and market its products, allowing airlines to make personalized offers and to sell ancillary products (like baggage fees, preassigned seats, boarding privileges, etc.) in the travel agency channel rather than only on its own website

nonchangeable ticket: a ticket that cannot be exchanged for a different route or flight once it’s been purchased

nonrefundable ticket: a ticket that cannot be returned for cash or credit once it’s been purchased but may be changeable for a fee

nonstop flightnot to be confused with a “direct flight”; a flight that goes directly from the origin to the destination without landing en route

open-jaw ticket: a return airline ticket where either the destination or the origin is not the same as the departing flight (e.g., a departing flight from New York to London with a return flight from Paris to New York, or a departing flight from New York to London with a return flight from London to Boston); a double open-jaw ticket is when the origin and destination of the return flight are both different as the departing flight (e.g., a departing flight from New York to London and a return flight from Paris to Chicago)

overbooking: a practice adopted by airlines where more seats are confirmed on a flight than available on the aircraft based on the assumption that there will be a few no-shows

PNR: Passenger Name Record; a record created in the GDS when a reservation is made that contains a traveler’s personal and itinerary information

reissue: when a new ticket must be created in exchange for another due to a change of plans; in most cases, a reissue will involve additional fees

stopover: not to be confused with “connection”; when a traveler changes planes to connect to another flight to reach their destination and the stop is more than four hours domestically or 24 hours internationally

yield management system: a sophisticated computer-based pricing system that airlines use to adjust prices based on anticipated demand