The Collaborative International Dictionary of English v.0.48:

anestrus \anestrus\ n.
   A state or interval of sexual inactivity between two periods
   of estrus; -- applies to nonhuman mammals. [Also spelled
   anoestrus.]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

anoestrus
    n 1: applies to nonhuman mammals: a state or interval of sexual
         inactivity between two periods of estrus [syn: anestrus,
         anestrum, anoestrus, anoestrum] [ant: estrus,
         heat, oestrus, rut]