A historian was someone who studied and wrote about the past.
Dojjen was a Manussan historian and Ambril's predecessor as Director of Historical Research. He believed that the Mara had not been destroyed and would someday return. He learned the rituals needed to defeat the Mara and taught them to the Fifth Doctor. (TV: Snakedance)
Tranlis Difarallio was a historian who studied the history of the Forrester family. He wrote From the Gutter to the Stars: A History of the Forrester Family. (PROSE: Dependence Day)
Kronos Vad was a famous historian who wrote a complete series on the history of Earth. Volume 36,379 played a key part in defeating the Gomagog. He was an acquaintance of Iris Wildthyme. (AUDIO: Kronos Vad's History of Earth Vol. 36,379)
Historians from an era that followed the Last Great Time War studied the history of the Dalek race, writing a book about the Dalek timeline from their origins on Skaro to their hypothesised final defeat in the Siege of Trenzalore. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)