A "flying ace" or fighter ace is a military aviator credited with shooting down five or more enemy aircraft during aerial combat. The term "ace in a day" is used to designate a fighter pilot who has shot down five or more enemy aircraft in a single day. Since World War I, a number of pilots have been honored as "Ace in a Day"; however, the honor of being the last "Ace in a Day" for the United States in World War II belongs to First Lieutenant Oscar Francis Perdomo of the 464th Fighter Squadron, 507th Fighter Group.