As defined by ABP "A Diplomate is a periodontist who has made significant achievements beyond the mandatory educational requirements of the specialty and who is certified by the American Board of Periodontology".
Graduate degree in the field of Periodontology from an accredited program in USA is mandatory before appearing for an oral and written examination administered by the American Board of Periodontology. The candidates who successfully complete the rigorous peer review process, are considered as Board Certified and conferred the title of Diplomate, American Board of Periodontology".
Having a diplomate status ensures that you are seeing a professional who has taken the time and effort to prove, before a panel of internationally respected peers, that he or she is competent enough to hold the specialties highest status.