Professional dominants are those who act in a Dominant role for a professional session in exchange for Tribute. There is no official formal training of professional dominants, nor certification. As such it is often wise to inquire about the techniques, mentors and training of someone that labels themselves as a professional dominant to ensure quality and safety, as well as to obtain references and referrals when possible.
The gender neutral term is often "Pro Dom" while the female specific term is often "Pro Domme", "Dominatix" or "Mistress" and the male pronoun is often "Master", though the specific Honorific chosen will often be left up to the pro dom in question and they may or may not follow this formula.
Culturally there are professionals that include some kind of sex work and those that strictly do not as part of their business. Always research, never assume and ask if you aren't sure about the policies of someone you are seeing professionally as well as applicable laws in your territory.