Originally Posted by Kiwi Flyer
Stopover means more than 24 hours based on flight schedule, as opposed to transit which is up to 23 hours 59 minutes.
Stopover can be any length pver 24 hours, subject to any limit on the ticket validity duration (and flight schedules).
the op asked about stopovers on DOMESTIC US awards, in which case the break point is usually 4 hours or the next flight out, whichever comes first.