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The Sith were not mentioned in the theatrical versions of the original Star Wars trilogy. However, Darth Vader was identified as "a Dark Lord of the Sith" as he made his first appearance in the novelization (officially authored by George Lucas, but actually ghostwritten by Alan Dean Foster). It also seems to imply that Vader's masked, mechanical appearance may not be unique to him, but may be instead be shared by all the Sith.
So who came up with the name "Sith," Lucas or Foster (or someone else)? And what was "Sith" conceived of meaning, when they were first named, ca. 1976?