Originally Posted by http://www.qantas.com.au/regions/dyn/au/specials/oneworldExplorer
...A stopover is where your onward travel does not take place within 24 hours...
Edited to add: The OWE starfiles contained this line for years
71N . 1. A STOPOVER IS A BREAK OF JOURNEY OVER 24 HOURS.
until 18 April 2006 when, for some perverse reason, it was removed from the rules.