A Nazi is a member of the National Socialist German Workers' party of Germany. This party was led by Adolf Hitler and in 1933 it seized political control of Germany turning this nation into a ideological dictatorship. The regime was characterized by the worshipping of Adolf Hitler as their leader or Führer, and the promotion of the master race, which led to the demise of jews, homosexuals, communists, gypsies, mentally handicapped, slavs and many other groups of people that didn't fulfill the ideal of the master race.
Modern-day Neo-Nazis think they had some pretty good ideas.
Nazi on internet discussion forums
Generally it is assumed that when one debater calls the opponent a Nazi or Fascist in an argument he has lost the debate. However, this principle is not valid if the opponent is indeed a Nazi or a Fascist.
An alternative but far more amusing way of terminating a discussion - never yet employed on Sciforums - is the 50 Hitler post and its variations.