Historian
Generally speaking, a historian is a person who studies history. This is harder than many think; the study of history requires careful attention to detail and neutrality. Historians can often have a profound impact upon the way people think. For example, Edward Gibbon's The History of the Decline and Fall of the Roman Empire revolutionized historical thought, and is often considered the first "modern" examination of history.
It should be noted that while "historian" can technically be used to refer to any person who studies history, it is generally only applied to people whose work is recognized in academia.
See also
de:Historiker
fr:Historien
ja:歴史家
simple:Historian
sv:Historiker
uk:Історик
zh:历史学家