What characterizes a "red-eye" flight?

A "red-eye" flight is primarily characterized as an overnight flight that departs late in the evening and arrives at its destination the following morning. This term refers to the fact that passengers often arrive with tired, red eyes due to lack of sleep during the travel. The scheduling of these flights is designed to allow travelers to maximize their time at their destination by flying overnight and effectively eliminating the need for a full day of travel. This characteristic is distinct from other types of flights, as it emphasizes the timing of departure and arrival along with the experience of the travelers.

The other options do not capture the essence of what defines a red-eye flight. For instance, a flight that departs during the day would not meet the overnight travel requirement, and a canceled flight due to weather does not pertain to the concept of overnight travel at all. Lastly, a flight with a high number of empty seats does not inherently describe the timing or nature of the flight itself, which is central to the definition of red-eye flights.