After graduating with a degree in veterinary medicine some individuals chose to specialize in a particular discipline through additional training.  The road to specialization most often involves a residency program and perhaps graduate studies. 

Following successful completion of a residency, credentialing and perhaps a few other requirements unique to individual specialties, a veterinarian is  eligible to sit a certification exam to complete requirements for  recognition and certification by a "specialty board". In some  specialties if a veterinarian hasn't completed a residency but has demonstrated required competence in the discipline from practice experience they may be permitted to sit the certification exam by completing a few additional (alternative) requirements.

Specialty colleges and boards are comprised of veterinarians with focused interests in specific disciplines (e.g.  American College of Veterinary Surgeons ,  American College of Veterinary Internal Medicine ,  American College of Veterinary Ophthalmology ,  American College of Theriogenologists , etc.) and are given the responsibility by the American Veterinary Medical Association  for training, testing and certifying other veterinarians as "specialists". Diplomate status in a specialty college or board gives a veterinarian recognition as having demonstrated special competence and expertise in the discipline in question.

The founding Diplomates of the ACT were a group of veterinarians that devoted much of their professional time and interest to the discipline of reproduction. They organized a specialty college to certify veterinarians as specialists in Theriogenology.  A Theriogenologist is a veterinarian who having demonstrated special competence in veterinary reproduction (male and female, large and small domesticated animals) is certified as a Diplomate (pronounced dip-low-mate) in the specialty college. The ACT is one of several specialty colleges recognized and sanctioned by the American Veterinary Medical Association .  Many ACT Diplomates focus their interests and activities on individual species (e.g., equine).

Only veterinarians with Diplomate status in a specialty college or board can legitimately refer to  themselves as a "specialist" or "board certified" though many veterinarians  limit the scope of their practice to specific disciplines and/or species. This designation protects animal owning consumers by assuring them that a veterinarian  with Diplomate status has demonstrated the competence required to legitimately call themselves a "specialist" and is  widely recognized as an expert in their respective discipline.

In other words, a Theriogenologist is a legitimate "reproduction specialist". A Theriogenologist that focuses on a particular species, such as equine, may refer to their self as an "equine reproduction specialist".