Definition: Secondary Source
Secondary sources are completely removed in proximity from the original event, person or place but seek to provide an interpretation based on primary sources. There is a continuum from primary to secondary sources, and many sources show elements of both.
Ken Burns's television documentary about the American Civil War is a secondary source that uses primary sources to tell a story. A memoir written by a Civil War veteran in 1887, twenty-two years after the end of the Civil War, can be considered a primary source, albeit far removed from the actual events. A history text about the Civil War written in 1987, however, is most definitely a secondary source.