There's no one thing, but for historians, it's generally something that was created at the time of the event. Try searching using terms like: biography [also use for autobiographies], newspapers, "personal papers" [use for archival material], correspondence [use for letters], personal diaries, photographs, videos, interviews, pamphlets, artwork, sources [a common one!], and many other types of things!

Historians not only reference the secondary sources they use, but they also document their primary works carefully. As such, you can use the bibliographies and notes to learn about specific titles of sources (Dahomey and Its Neighbors) as well as types of sources (travel accounts, early treatises, letters) that historians use to explore a subject.