es·trus also oes·trus 
(ĕstrəs)
(ĕstrəs)n.
The periodic state of sexual excitement in the female of most mammals, excluding humans, that immediately precedes ovulation and during which the female is most receptive to mating; heat.
[New Latin, from Latin oestrus, frenzy, gadfly, from Greek oistros; see eis- in the Appendix of Indo-European roots.]