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!

There is no way to limit search results to "primary sources" in Catalyst, but there are a couple of ways to identify primary sources on a results page if you know the type of primary sources you are searching for.

Using Catalyst  main search box:

• Type in your keyword and hit search
• On your results page, on the left-hand side, browse through "Material Types" and "genre/form."

Using Catalyst advanced search:

• Using the Catalyst "advanced search," type in your KEYWORD/S and limit to "subject." 
• In the second search box below, add additional terms such as "personal narratives," "memoirs," or "maps," depending on the type of primary source you are hoping to find.  Limit this second search box to "genre." For examples, you can search for Roman Empire (subject) and maps (genre).

• Do an author search for persons, organizations, or group central to your topic.
  • Author/Creator (contains) Bolívar, Simón
  • RESULT: The Bolívarian Revolution (English translations of letters and other selections)

• Do an advanced search for words that signify primary source (source, letters, narrative, laws) and words that represent your topic
  • Subject (contains) Cuba History sources
  • RESULT: Voices of the enslaved in nineteenth-century Cuba : a documentary history (translation of original into English)
  • PROTIP: just adding History Sources onto the end of a country can often get good results!

• Do a subject search using a subject term you've discovered while searching. Refine your search to publications produced during the era you are researching.
  • Subject (is exact) Brazil--Commerce, Sort by Date - Oldest 
  • Report of the commissioners from British North America appointed to inquire into the trade of the West Indies, Mexico & Brazil (1866)