A domestic flight is one that takes place entirely in one country; both the departure and arrival cities are in the same country. For example, if your itinerary includes a flight from the US to Paris, and then a flight from Paris to Nice, the Paris–Nice flight is the domestic one.
If you’re a US citizen taking a domestic flight within the US, you do not need a passport. You will need an approved form of identification to fly, though. These may include a driver’s license or other state-issued ID card, a permanent resident card, or a government-issued passport. There are provisions for travelers who have lost their identification, however, so if you lose your ID right before your flight, you can still fly.
The truth is that it varies, in part depending on how busy the airport will be and how much you need to do before you can go to the gate.
During peak travel seasons, airports are much more crowded, security lines are longer, and you’ll need to allow more time. The same goes for particularly busy times of day, such as Monday mornings (when business travelers are starting their work week). And, if you’re checking a bag, that requires more time than simply walking directly to security with a carry-on.
As a general rule, however, if you’re not sure whether it’s a busy time of year or day, most airlines suggest getting to the airport roughly two hours ahead of domestic flights. During busy travel periods, arriving three hours in advance of your flight isn’t a bad idea.
If you’re traveling when you know the airport will be less busy, it’s likely you can safely get to the airport with less of a time cushion—but keep in mind that each airline has a cut-off time by which either checked bags must be dropped off or passengers must be checked in (or both). Your arrival time, therefore, should be based on that cut-off time rather than the time your flight is scheduled to depart.
Again, this one varies significantly depending on the airport and your itinerary. Each airport has a minimum connection time (airlines aren’t allowed to book you on a connection under this time limit) which is often posted on its website. For most airports, 30–45 minutes as a good general baseline, but again the minimum viable connection time that’s wise for your individual trip can vary based on a lot of factors, like how likely your first flight is to be delayed and how quickly you can maneuver through an airport (e.g. traveling with your elderly parents? You might want more time.)
Note that larger airports, or even smaller airports with both domestic and international flights, will require more time to navigate. If your connection is between two domestic flights, that may be a quick trip from one end of a concourse to the other. If your connection is between a domestic and an international flight, however, you may need to go to an entirely different terminal. And of course, if you’re coming from an international flight and connecting to a domestic one, you’ll need to allow time for going through customs and immigration.
If your trip is two different itineraries (i.e. you booked a cheap international flight to get to Europe and then booked a separate flight to get to the city you really want to be visiting), you may need to go through baggage claim and then security all over again—and that will take even more time. In those cases, it’s wise to allow a few hours for connecting.