Primary sources are original materials on which further research is based- the firsthand accounts for history books or the experimental data and initial reports for scientific reviews.
In the sciences & engineering, primary sources are documents that provide full description of the original research. An original publication of a scientist's new data, results and theories, experimental research results in scientific journals.
In the social sciences, any original document or firsthand report such as a diary, paper or tape recording that provides data for analysis.