When a monster is discarded, is it considered "defeated"?
Page 8 of the Reference Guide


  Monster
  
  When a Monster has lost Health equal to or greater than its toughness, it is defeated.
  
  When a Monster is defeated or discarded, it is returned to the Monster cup and the cup is randomized.


If you have to encounter something (such as an ambush) on a temporary basis and are able to do enough damage to it in one go to kill it, then it is considered 'defeated'.  However, it does make a distinction here about both 'defeated' and 'discarded'.  This means that, as far as I can tell, there is a precedence about the difference between the two.  So, no, discarding does not equal defeated.