A primary source is a document or other sort of evidence written or created during an event or time of study (or after the event, if done by an
eyewitness). Primary sources offer an inside view of a particular event.
In the field of law, primary sources are the actual text of the law, that is, legislation, opinions, case law.
Other types of primary sources: diaries, speeches, official records, scholarly journal articles reporting new research or findings.
Image source: Kelo v. City of New London. 2005. US Supreme Court website.