Oestrus

from 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}]

from 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]