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A primary source can have a different definition depending on which discipline you are in and depending on your research question.
In the sciences, primary sources are scientific journal articles that report the results of research studies.
In other fields, such as history, primary sources are those sources that were created at the time of an event. These include photographs taken of the event, newspaper articles written during the time of the event, diary entries, letters, and ephemera also from the time of the event.
So, if you were doing research around World War II, newspaper articles written during WWII would be considered primary sources, but a book written in 2000 describing WWII would not be considered a primary source.
When in doubt, ask a librarian or your instructor.
Finding Primary Sources Online
Primary sources can be harder to locate online than other types of sources, but they can be easier to identify.