Fuehrer when the ü-umlaut is not used, but never just Fuhrer) is a proper noun meaning "leader" or "guide" in the German language. The IPA transcription of the standard German pronunciation is [ˈfyːʀəɐ], but in English it is usually pronounced [ˈfjʊːɹə(ɹ)]. It is mainly used in English for Nazi Germany's absolute ruler Adolf Hitler. Though the word remains common in German, it does come with some social stigma attached, and under Hitler was part of the title of many positions in the various (para)military and governmental organizations of Germany.
Führer was the title granted by Hitler to himself by law, as part of the process of Gleichschaltung, following the death of the last Reichspräsident of the Weimar Republic, Paul von Hindenburg, on August 2, 1934. The new position, fully named Führer und Reichskanzler (Leader and Chancellor of the (Third) Reich), unified the offices of President and Chancellor, formally making Hitler Germany's Head of State as well as Head of Government respectively; in practice, the Dictator of the Nazi Third Reich.
Nazi Germany cultivated the Führerprinzip (leader principle), and Hitler was generally known as just der Führer ("the Leader"). One of the Nazis' most-repeated political slogans was Ein Volk, Ein Reich, Ein Führer' - 'One people, One state, One leader'.
For military matters, Hitler used the style Führer und Oberster Befehlshaber der Wehrmacht ('Leader and Supreme Commander of the Wehrmacht'), until that addition was dropped in May 1942 by decree of the Führer. The style of the Head of State for use in foreign affairs was Führer und Reichskanzler (Leader and National Chancellor) until July 28, 1942, when it was changed to Führer des Grossdeutschen Reichs ('Leader of the Greater German Nation').
Führer has been used as a military title (compare Latin Dux) in Germany since at least the 18th century. Ironically, given the context of the word to refer to Adolf Hitler as supreme ruler of Germany, in the context of a company sized military subunit in the German Army, the term "Führer" referred to a commander lacking the qualifications for permanent command. For example, the commanding officer of a company was titled "Kompaniechef" (literally, Company Chief in English), but if he did not have the requisite rank or experience, or was only temporarily assigned to command, he was officially titled "Kompanieführer." Thus operational commands of various military echelons were typically referred to by their formation title followed by the title Führer, in connection with mission-type tactics used by the German military. The term Führer was also used at lower levels, regardless of experience or rank; for example, a Gruppenführer was the leader of a squad of infantry (9 or 10 men). Aside from this generic meaning, "Gruppenführer" was also an official rank title for a specific grade of general in the Waffen SS. The word Truppenführer was also a generic word referring to any commander or leader of troops, and could be applied to NCOs or officers at many different levels of command.
Under the Nazis, the title Führer was also used in paramilitary titles (see Freikorps). Almost every Nazi paramilitary organization, in particular the SS and SA, had Nazi party paramilitary ranks incorporating the title of Führer.