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, which is designated between two points. It can start and end at any point or 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 in which the plane turns from the outbound course to the inbound course. Then there is the inbound course segment, which is the point in 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 the 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 one flight will have all these segments and in many cases they will have more then one of each. For example: a step climb is comprised of many climb and cruise segments. It’s where air-traffic control clears and airplane up to a specific altitude for a period of time and 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 portion of a flight route designated by two points, usually a fix or navigational aid, as well as being bounded in 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 in 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.