Template:Secondary source
Secondary sources, in the study of history, are those writings which were not penned contemporaneously with the events in question.
For example, if an author reads a history book and then uses the information in that book as a source for writing a new history of the same events, the author is using a secondary source.
If, however, the author uses writings by participants and contemporaries--people who have personal, first-hand knowledge of the events in question--the author is using primary sources to write history.