Webster's English Dictionary

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estrus

es.trus or es.trum \'es-tr*s\ \-tr*m\ n [NL, fr. L oestrus gadfly, frenzy, 
   fr. Gk oistros - more at IR]E 1a: a regularly recurrent state of sexual 
   excitability during which the fem ale of most mammals will accept the male 
   and is capable of conceiving : HEAT 1b: a single occurrence of this state  
   2: ESTROUS CYCLE