expert
person with broad and profound competence in a particular field
expert witness
someone with specialised knowledge who testifies in court
connoisseur
subject-matter expert
subject expert
person who has specialist domain knowledge on a particular subject
peritus
invited expert at a church council
guide
person, who with knowledge or wisdom of the subject matter, escorts others, e.g., travelers or tourists through unknown or unfamiliar locations
exhibition curator
person in charge of organising an exhibition
local expert
person with an intimate knowledge of a region
methodologist
practicer of methodology
negotiator
person who negotiates
chess master
very good chess player
thought leader
entity that sets societal trends of thought in a particular context
power user
computer user who uses more advanced features of computer hardware, operating systems, programs, or web sites than the average user
maestro
honorific title of respect given to a master musician; most often used in addressing or referring to conductors
gourmand
person who takes great pleasure in food