A person appointed as a guide in a specific area. To be appointed the person must (a) possess superior knowledge in that specific area, and (b) be trusted to use their superior knowledge in a fair or better (i.e., generous to selfless) way.
Note that a true ‘authority figure’ is appointed as such, which promotes an harmonious relationship. An imposed (rather than appointed) guide is actually a ‘power figure’, which often invites a conflict.