1. A recurrent, restricted period of sexual receptivity in female mammals other than humans, marked by intense sexual urge as a result of congestion of and secretion by the uterine mucosa, proliferation of vaginal epithelium, swelling of the vulva, ovulation, and acceptance of the male by the female.

During estrus, the animal is said to be "in heat".

This word is presented here to let you know that it has no etymological connection with the estiv-, aestiv-; estuar-, aestuar- elements of this unit. Most dictionaries do not make this distinction and that is why it is presented here.

2. Etymology: from Latin oestrus, "frenzy, gadfly"; from Greek oistros, "gadfly, breeze, sting, mad impulse; frenzy".