A historian is a person who researches, studies, and writes about the past, and is regarded as an authority on it. Historians are concerned with the continuous, methodical narrative and research of past events as relating to the human race; as well as the study of all history in time. If the individual is concerned with events preceding written history, the individual is an historian of prehistory. Although "historian" can be used to describe amateur and professional historians alike, it is reserved more recently for those who have acquired graduate degrees in the discipline. Some historians, though, are recognized by publications or training and experience. "Historian" became a professional occupation in the late nineteenth century as research universities were emerging in Germany and elsewhere. 

During the "Irving v Penguin Books and Lipstadt" trial, it became evident that the court needed to identify what was an "objective historian" in the same vein as the reasonable person, and reminiscent of the standard traditionally used in English law of "the man on the Clapham omnibus". This was necessary so that there would be a legal bench mark to compare and contrast the scholarship of an objective historian against the methods employed by David Irving, as before the "Irving v Penguin Books and Lipstadt" trial, there was no legal precedent for what constituted an objective historian.
what do you need to have now to regarded as a historian these days?
 it is reserved more recently for those who have acquired graduate degrees in the discipline. 