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!
The Center for Research Libraries has a number of microfilmed collections, international newspapers, and official documents from countries outside the U.S.
Questions? Contact firstname.lastname@example.org
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