In the sciences, a primary source:
Primary scholarly references are the gold standard for your background research as a scientist. Secondary scholarly literature—review articles, books, encyclopedias, handbooks, etc.—are useful entry points, but shouldn't be used alone. Follow up on the citations you find in secondary sources to get to the primary scholarly references.
Scientific primary literature is peer reviewed, or refereed, before being published. This enhances the quality and validity of the work. In the peer-review process, two or three specialists in the field read and critically evaluate the work before it can be published. Peer review is a quality-control measure to ensure that the primary literature includes only high quality, valid scientific information. Primary authors may revise and resubmit articles to improve them. Secondary literature is less stringently reviewed.
Modern/Current Newspapers
In humanities disciplines, including history, a primary source: