The term "on heat" is often used to describe a female animal's readiness to mate. However, there are several other synonyms that can be used to convey the same meaning. "In estrus" is a scientific term used in veterinary medicine and biology. "In season" is a commonly used phrase to describe the time when an animal is in heat. "In heat" is another way to describe this state of being. Other terms used less frequently include "in rut" and "harmonal." While these words may not be as commonly used, they offer alternative options for describing an animal's reproductive cycle.