Oestrum, often spelled as "estrus," refers to the period when animals are sexually receptive and able to mate. This term is commonly used in the animal kingdom, especially among mammals. However, there are several synonyms for oestrum, such as heat, rut, and breeding season. Heat is usually used to describe female animals during their oestrum period, while rut pertains to male animals looking for mating opportunities. Breeding season, on the other hand, is a more general term that refers to the time of year when animals mate. These synonyms are helpful for animal researchers, breeders, and owners to understand the nature of animal reproduction and behavior.