People who do their level best to create or obtain everything appropriate to their persona. They can be, and are, frequently a wonderful resource. The SCA requires an attempt at periodicity--how far you want to, or can, take it is up to you. Don't be afraid to ask questions.
The term is occasionally derogatory, along with the always derogatory and offensive "Authenticity Nazi." (We really try to dissuade use of the latter term, for obvious reasons.) Here it means that someone likes to take things apart in ways not befitting a courteous person. As a friend once noted, "Saying 'That's not period!' isn't period."
-- Mistress Arlys o Gordon