The words that decide what you are owed.

The DOT’s monthly report on airline performance and consumer complaints, and the source of the official denied-boarding and complaint tables.

One of the causes the DOT assigns to a delayed flight. It means the aircraft due to operate your flight arrived late from its previous trip.

It is a propagation category, not a root cause. A carrier with a lot of it has a schedule that does not absorb disruption, but the original problem happened somewhere earlier in the day.

Any charge for something other than the seat itself: checked bags, seat selection, change fees, priority boarding, wifi.

Under 14 CFR 260.4 a fee for a service you paid for but did not receive is refundable.

A flight that was published in the schedule and did not operate.

Cancelled flights sit outside the denominator for average delay minutes, which means a carrier can improve its delay average by cancelling its worst flights. Always read the two figures together.

The range around an estimate that is compatible with the data. When we publish "2.32% (95% CI ±0.065 pts)", the plausible range is roughly 2.26% to 2.39%.

If two airlines’ intervals overlap, the honest description is that they are statistically indistinguishable, not that one is worse. The margin on a difference between two numbers is roughly twice the margin on either one.

What the DOT began reporting in mid-2023 in place of a straight complaint count. It bundles complaints together with inquiries and opinions.

This is a definitional break. Raw year-over-year complaint totals across mid-2023 are not comparable, because the thing being counted changed.

The airline’s own terms of sale: the document that governs your ticket beyond what federal rules require.

It can give you more than the regulations do, never less. It is also where airlines put the limits people are most surprised by.

The cash payment owed when an airline involuntarily denies you boarding on an oversold flight, calculated from your fare and how late the airline gets you there.

The ceilings are $1,075 and $2,150. Those are maximums, not typical payments.

A flight that landed somewhere other than its scheduled destination.

Reported separately from both delays and cancellations, so it appears in neither of those rates.

One passenger boarding one aircraft at one airport.

A connecting passenger is counted at every airport they board from, so enplanements cannot be summed into a national passenger total.

The financial and traffic reporting airlines file with the DOT. It is where ancillary fee revenue, including baggage and change fees, comes from.

Being refused a seat on an oversold flight you hold a confirmed reservation for, without volunteering to give it up.

The distinction from volunteering is what matters: if you accept an offer to give up your seat, the rule’s minimum payments no longer apply and whatever you negotiated is the deal.

For DOT reporting, a flight that arrives at the gate 15 minutes or more after its published arrival time.

Fifteen minutes is a reporting definition, not a rights threshold. Nothing is owed at 15 minutes; the refund threshold is three hours domestic.

The share of available seats that were actually occupied.

A high load factor is why oversold flights are hard to rebook: there is nowhere to put you.

The airline whose code is on your ticket, whoever actually operates the aircraft.

From 1 January 2018, DOT reportable flights are attributed on the marketing-carrier basis. Before that they were attributed to the operating carrier, which is why pre- and post-2018 airline comparisons describe different populations.

A checked bag that is lost, delayed, damaged or pilfered, as reported to a carrier by or on behalf of a passenger.

Note "as reported". A mishandled-bag rate is a rate of reports, not of events, so it undercounts anything passengers do not bother reporting.

The treaty governing airline liability on international itineraries, including for baggage and delay.

It applies instead of the US domestic baggage limit on international trips, and its limit is set in a currency unit rather than dollars.

For DOT reporting, a flight that arrives less than 15 minutes after its published arrival time.

The airline that actually flies the aircraft, which is often a regional carrier flying under a major’s code.

The counterpart to marketing carrier, and the basis DOT used before 2018.

Selling more confirmed seats on a flight than the aircraft has.

Legal, and routine. What is regulated is what the airline owes you when the oversale results in your being denied boarding.

The unit for the difference between two percentages.

A move from 20% to 22% is two percentage points, not two percent. The distinction matters because "two percent" would describe a move to 20.4%.

A domestic non-stop scheduled passenger flight, held out to the public under a reporting carrier’s code, to or from a US large, medium, small or non-hub airport.

An airline large enough that the DOT requires it to file on-time performance data. Since 1 January 2018 the threshold is half of one percent of domestic scheduled passenger revenue; before that it was one percent.

The set is not fixed. Carriers enter and leave it as their revenue share crosses the threshold, and some file voluntarily. So "all airlines" in a DOT table means all reporting airlines.

A departure or arrival moved by three hours or more on a domestic itinerary, or six hours or more internationally. Also covers a change of airport, added connections, a cabin downgrade, and loss of an accessibility feature.

This is the threshold that turns a delay into a refund right. Below it, federal rules provide no payment for the delay itself.

A checked bag not delivered within 12 hours of the last flight segment arriving on a domestic itinerary, or within 15 or 30 hours internationally depending on the length of the longest non-stop segment.

Crossing the threshold makes the checked-bag fee refundable. It is separate from any claim for the contents.

Time spent on the ground in an aircraft with passengers on board, either before takeoff or after landing.

Part 259 requires carriers to have a plan with hard time limits and to let passengers off past them.

Giving up your seat on an oversold flight in exchange for whatever the airline offers.

The regulated minimums in 14 CFR 250.5 do not apply once you volunteer. The offer is a negotiation, and the airline has every incentive to settle before it becomes involuntary.