A historian is a person who, either as a professional or an amateur, researches events in the past and present and interprets those events as neutrally and accurately as possible in a scientific manner. A historian works in the field of history.
A team of 27th century historians, led by Amila Thon, travelled back in time aboard their alien ship to observe the Battle of Wolf 359 in late 2366 and early 2367. Six of their team went to the Borg cube and were most likely assimilated. (Star Trek: The Prospect Chronicles: "The Burnt Child")