Flights can get delayed for a variety (and often a combination) of reasons and knowing the lingo helps manage expectations and when seeking a refund or compensation. “It [delays] could be technical, operational, crewing, weather, air traffic control, or waiting for connecting passengers,” says Brett Manders, pilot and author of Behind the Flight Deck Door.
Operational: "Sometimes busy airports can be constrained by the number of arrivals and departures they can process."
Crewing: "On occasions, the crew can 'run out of hours.' Pilots and cabin crew have very specific limits on the number of hours we are allowed to work. Whilst pilots have been called glorified bus drivers, and people think cabin crew is airborne waiters/waitresses, we actually have roles related to safety. You don’t want your crew suffering from fatigue should an emergency happen."
Weather: "Thunderstorms in and around an airport really put the brakes on operations. Pilots don’t want to fly near them. An airport could have multiple runways, but due to strong winds, only one is open for take-offs and landings. This will slow down departures and arrivals. It will also cause consequential delays down the line for airports that aren’t affected by the weather system."