Primary sources are the objects of study. The definition of a primary source changes dependent on your application, and interpretation of the source. A newspaper article, for example, can be a secondary source (an interpretation of an event), if you are reading it in lieu of seeing the event. Or it can be a primary source if you are researching other people's interpretations of the event, e.g., how an event was seen in the past.
The following terms are found in the subject headings of records for published primary sources.
Include one of these terms in with your search, e.g., immigrants correspondence. There are lots of ways to play with these terms, if you have questions or want to learn more, ask Sarah (firstname.lastname@example.org)!