Formal:
* Expert: Highly knowledgeable and skilled in a particular area.
* Professional: Someone engaged in a specific profession, often requiring training and certification.
* Practitioner: Someone who regularly practices a particular skill or profession.
* Technician: Someone with specialized knowledge and skills in a technical field.
* Consultant: Someone who provides expert advice in a specific area.
* Guru: A recognized authority or leader in a particular field (often used informally).
Informal:
* Pro: A professional or expert, often used in sports or entertainment.
* Ace: Someone who excels in a particular skill or area.
* Whiz: Someone exceptionally skilled or knowledgeable in a specific field.
* Master: Someone who has achieved a high level of skill or knowledge.
* Wizard: Someone with exceptional abilities in a particular area (often used metaphorically).
Specific to fields:
* Physician: A medical doctor.
* Surgeon: A doctor specializing in surgery.
* Therapist: A healthcare professional who treats mental or physical conditions.
* Engineer: A professional trained in engineering.
* Scientist: A professional trained in science.
* Lawyer: A professional trained in law.
More general terms:
* Authority: Someone who is recognized as having expertise in a particular area.
* Connoisseur: Someone who is a discerning judge of quality, especially in art, food, or music.
* Specialist: This is the most general term, and can be used for any professional who focuses on a specific area.
The best word to use will depend on the specific context and the desired tone.