A segment is a non-stop point to point flight. You can have a multi-segment flight on the same aircraft with the same flight number. However, increasingly some US carriers have created multi-segment flights using the same flight number but including a change of aircraft. Gets very confusing. Also, on most segment bonuses, en route stops do not count as multiple segments. As confusing as why the terms "direct" and "nonstop" get used improperly, where the former may include several stops but no change of aircraft or flight number (i.e. multi-segment) and the latter exactly as it sounds, no stops from origin to destination.