(covers information from several alternate timelines)
Führer (or Fuehrer) was a word from the German language which translated as leader.
Adolf Hitler was the Führer of Nazi Germany in the mid-20th century. When John Gill reformed Ekosian society after Nazi principles in the 2260s, he also took the title of Führer. (TOS: "Patterns of Force")
In a holoprogram set in 1944 France, a Nazi officer told Turanj that Karr had never embraced the Führer or his vision and they were therefore right to disobey his orders. (VOY: "The Killing Game, Part II")
In an alternate timeline 1944, Hitler was referred to both as the Führer and German chancellor in News on Parade. The title was also used by a German general and Vosk when speaking about Hitler. (ENT: "Storm Front, Part II")