Definition (Source: Wikipedia)
*** A veterinary specialist is a veterinarian who specializes in a clinical field of veterinary medicine.
"A veterinary specialist, as recognized by the AVMA, is a graduate veterinarian who has successfully completed the process of board certification in an AVMA-recognized veterinary specialty organization (ie, board or college). To become board certified, a veterinarian must have extensive post-graduate training and experience and pass a credential review and examinations set by the given specialty organization." [1]
Most specialties require a 1-year internship or 2 years of clinical practice prior to beginning a residency of 2-3 years' duration. Most specialties require the resident to produce some academic contribution (often in the form of a scientific publication) in order to qualify to sit the certifying examination.