The Collaborative International Dictionary of English v.0.48:

oestrus \oes"trus\, n. [L., a gadfly; also, frenzy, fr. Gr. ?
   gadfly; hence, sting, fury, insane desire, frenzy.]
   1. (Zool.) A genus of gadflies. The species which deposits
      its larvae in the nasal cavities of sheep is oestrus
      ovis.
      [1913 Webster]

   2. A vehement desire.
      [1913 Webster]

   3. Specifically: (Physiol.) The periodical sexual impulse of
      animals; the period during which female animals are most
      receptive to males; heat; rut.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

oestrus
    n 1: applies to nonhuman mammals: a state or period of
         heightened sexual arousal and activity [syn: estrus,
         oestrus, heat, rut] [ant: anestrum, anestrus,
         anoestrum, anoestrus]
    2: type genus of the Oestridae: sheep botflies [syn: Oestrus,
       genus Oestrus]