The Collaborative International Dictionary of English v.0.48:

diestrus \diestrus\ n.
   a state or interval of sexual inactivity or quiescence
   between periods of activity; -- of animals having several
   estrous cycles in one breeding season.

   Syn: diestrum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

diestrus
    n 1: (of animals having several estrous cycles in one breeding
         season) a state or interval of sexual inactivity or
         quiescence between periods of activity [syn: diestrus,
         diestrum]