A flight segment refers to any single takeoff and landing by an aircraft. Essentially, it’s an undivided trip between two cities with a single flight number.
A flight segment is often used interchangeably with the term flight leg. A flight leg is also a trip from the origin to the destination airport, but it never has an intermediate stop. On the other hand, a flight segment can have a stop, for example, for refueling, when passengers don’t leave the plane. In this case, a flight segment consists of two legs.