The phrase "be in heat" refers to the period of reproductive cycle for animals, during which they become sexually active and ready to mate. There are several synonyms for this phrase, including "in estrus," "in season," and "in rut." These terms are commonly used among veterinarians, animal breeders, and hunters. Additionally, the phrase "on heat" is a term commonly used in Britain to describe the same phenomenon. Each of these phrases has been used to describe animals such as dogs, cats, horses, and deer. It should be noted that while these phrases are often used interchangeably, they may refer to different stages in the reproductive cycle of specific animals.