Estrus is a phase of the estrous cycle, which is a reproductive cycle in females of most mammalian species that is different from the menstrual cycle found in humans and other animals such as monkeys or bats. The estrus phase of the estrous cycle refers specifically to the period when the female is ready to have sexual intercourse for reproductive purposes, also known as being "in heat". The entire estrous cycle of most mammalian females is different from the human menstrual cycle in the sense that the lining of the uterus, which is called the endometrium, is not removed through a periodical bleeding discharge, but is rather processed and absorbed in the body without visible external signs.

Unlike human females, most mammalian species have periods of reproductive rest, also called anestrum, that involves the absence of the estrous cycle and thus inability to conceive offspring. For example, goats, cows, or horses are sexually receptive only a few times per year, which largely depends on the seasonal changes and on the availability of food. Humans, rabbits and other species are sexually active during the entire year.

The estrus phase specifically refers to the second phase of the estrous cycle, which is observed after a preparatory phase called proestrus. During estrus, the ovarian follicles stimulated by hormones released from the pituitary gland, reach their highest level of maturity, and the estrogen secretion, which is the female sexual hormone, reaches its peak. This is the period when mammalian females are fertile and sexually receptive. Most females may exhibit visible signs when they enter the estrus phase such as specific smells, behaviors and physical signs such as reddening of the labia or certain specific reproductive reflexes that enable the males to recognize the "heat" period. If conception will not occur during this phase, the estrogen levels will drop and the lining of the uterus will be re-absorbed without external vaginal discharge.