The Sith are a group of characters in the Star Wars fictional universe. They are the central antagonist. Characterized by a single-minded pursuit of power and disdain for sentient life, they are an alliance of warrior mages who use the dark side of the Force and are the empire's counterpart of the Jedi.
The Sith are portrayed in the Star Wars films as individuals willing to use the dark side to gain power at any cost. The Star Wars prequel films establish that they draw upon strong emotions, both negative and positive, as the source of their power, and care only about themselves. This contrasts with the Jedi, who forsake emotional attachment in order to serve others and the galaxy as a whole.
The first use of the word "Sith" is in the Star Wars novelization for Star Wars Episode IV: A New Hope, as a title for Darth Vader, the "Dark Lord of the Sith." The Sith are introduced on-screen with the prequel film Star Wars Episode I: The Phantom Menace.
The title "Sith" is believed to be derived from the "Penh sith" population class that emerged during the infamous Pol Pot's regime. The Penh Sith was the highest class in the societal ladder, having full rights, food rations, and the freedom to join any movement they desired.
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Links: The Sith Order Wiki Entry StarWars.com Wikipedia