The fiancé(e) K-1 nonimmigrant visa is for the foreign-citizen fiancé(e) of a U.S. citizen. The K-1 visa permits the foreign-citizen fiancé(e) to travel to the United States and marry his or her U.S. citizen sponsor within 90 days of arrival.
The foreign-citizen will then apply for adjustment of status to a permanent resident (LPR) with the Department of Homeland Security (DHS), U.S. Citizenship and Immigration Services (USCIS). Because a fiancé(e) visa permits the holder to immigrate to the U.S. and marry a U.S. citizen shortly after arrival in the United States, the fiancé(e) must meet some of the requirements of an immigrant visa. Eligible children of K-1 visa applicants receive K-2 visas.
You can learn more about fiancé(e) visas on usvisas.state.gov.