Is a K1 visa a nonimmigrant or immigrant visa?

Under U.S. immigration law, a foreign-citizen fiancée of a U.S. citizen is the beneficiary of an approved I-129F Petition for Alien Fiancée. A beneficiary who has been issued a nonimmigrant K-1 visa for travel to the United States in order to marry his or her U.S. citizen fiancée. During the process the word immigrant is also used for the K1 Visa process but still it is recognized by U.S. immigration as nonimmigrant.