expositor       
noun [ C ]  formal uk/ɪkˈspɒz.ɪ.tər/ us/ɪkˈspɑː.zə.t̬ɚ/
 说明者,解释者,阐述者;评注人
a person who explains or describes something, especially complicated ideas
  He regarded himself primarily as an expositor and defender of Platonic philosophy. 他认为自己首先是柏拉图哲学的解释者和捍卫者。 
 - More examples
 - He is arguably the most influential contemporary expositor of modern science.
 - Watt has long been acknowledged to be among the finest and most learned of Conrad's expositors.
 - He used his well-known ethics course partly to develop his own ideas, but was most noted as a critic and expositor.
 - Alf van der Poorten is one of Australia's leading expositors on number theory.