Each academic discipline creates and uses primary and secondary sources differently. The definition of a primary source only makes sense in the context of a specific discipline or field of inquiry.
Here are two definitions that try to capture the elusive nature of primary documents.
A definition from Cornell University:
"Primary sources are the main text or work that you are discussing (e.g. a sonnet by William Shakespeare; an opera by Mozart);
actual data or research results (e.g. a scientific article presenting original findings; statistics); or historical documents (e.g. letters, pamphlets, political tracts, manifestoes)."
["What is a Source?" Recognizing and Avoiding Plagiarism. Cornell University. College of Arts and Sciences.]
A definition from Yale University:
"What are primary sources? Primary sources provide firsthand testimony or direct evidence concerning a topic or question under investigation.
They are usually created by witnesses or recorders who experienced the events or conditions being documented. Often these sources are created at the time when the events or conditions are occurring, but primary sources can also include autobiographies, memoirs, and oral histories recorded later."
[Primary Sources at Yale. Yale University.]
Also on this site: Primary Sources come in all shapes and sizes.
Secondary sources are books, periodicals, web sites, etc. that people write using the information from primary sources. They are not written by eyewitnesses to events, for instance, but use eyewitness accounts, photographs, diaries and other primary sources to reconstruct events or to support a writer's thesis about the events and their meaning. Many books you find in MavScholar are secondary sources.
However, some publishers reformat primary-source documents and publish them in book format; for example, see The American Revolution: A History in Documents.
Tertiary sources are publications that summarize and digest the information in primary and secondary sources to provide background on a topic, idea, or event. Encyclopedias and biographical dictionaries are good examples of tertiary sources.
[New York]: McGraw-Hill.
[New York]: Oxford UP, 2000- .
2nd edition. Amsterdam: Elsevier, 2015- .
New York: Oxford UP, 2013.
New York: Oxford University Press, 2013.