A segment can be one of many things. It can be a holding segment, a flight segment, or more commonly, a route segment. It is defined as a portion of time, distance, or flight that is designated between two points. Furthermore, it can start and end at any point in time after takeoff and before landing.
A holding segment is one of four phases of holding. Holding is done over a fix or particular point. First, there is an outbound course segment, which is the period of holding in which the plane is flying away from the fix. Next, there is the inbound turn segment, which defines the point at which the plane turns from the outbound course to the inbound course. Then there is the inbound course segment, which is the point at which the plane is flying in the direction of the fix or point. Finally, there is the outbound turn segment, which is the point where the plane turns towards the fix and to the outbound course. These segments are defined either by air-traffic control, the FAA, or by aviation publications and give dimensions and/or restrictions as per the length, time, or speed of the segments.
The flight segment can be defined as any point of the flight in which the plane is transitioning from takeoff to just before landing. It can consist of the departure segment, climb segment, cruise segment, approach segment, or descent segment. For the most part, any flight will have all these segments and, in many cases, they will have more than one of each. For example, a step climb comprises many climb and cruise segments. It’s where air-traffic control clears an airplane up to a specific altitude for a period of time, then clears them up to a higher altitude for a period of time, and so on until the airplane reaches its final cruise altitude to its destination.
A route segment is a section of a flight path that is defined by two points, usually a fix or navigational aid, and is bounded between them. An example of this would be the segment of flight in between two VORs (very high frequency omni-directional radio range).
Segments can also be defined as a point in time at which the plane has a specific amount of fuel left with respect to the total time and/or distance in flight or the time and/or distance of flight from the point of origin.