A primary source is an original object or document that provides first-hand information about a topic or event. Primary sources are created by people or things that were there at the time or event, and they offer original thought and have not been modified by interpretation. Examples of primary sources include historical and legal documents, eye witness accounts, results of an experiment, statistical data, pieces of creative writing, and art objects.

Primary sources are distinguished from secondary sources, which are accounts that retell, analyze, or interpret events, usually at a distance of time or place. Secondary sources often quote or otherwise use primary sources, and they can cover the same topic, but add a layer of interpretation and analysis. Examples of secondary sources include most books about a topic, analysis or interpretation of data, and scholarly or other articles about a topic, especially by people not directly involved.

The definition of a primary source varies depending on the academic discipline and the context in which it is used. In the humanities, a primary source could be defined as something that was created either during the time period being studied or afterward by individuals reflecting on their involvement in the events of that time. In the social sciences, the definition of a primary source would be expanded to include numerical data that has been gathered to analyze relationships between people, events, and their environment. In the natural sciences, a primary source could be defined as a report of original findings or ideas.