Source : Webster's Revised Unabridged Dictionary (1913)

OEstrus \[OE]s"trus\, n. [L., a gadfly; also, frenzy, fr.Gr. ?
   gadfly; hence, sting, fury, insane desire, frenzy.]
   1. (Zo["o]l.) A genus of gadflies. The species which deposits
      its larv[ae] in the nasal cavities of sheep is {[oe]strus
      ovis}.

   2. A vehement desire; esp. (Physiol.), the periodical sexual
      impulse of animals; heat; rut.

Source : WordNet®

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