The movie Star Wars: Revenge of the Sith is all about a Sith Lords who wants to take over the control of the Republic and the Galactic Order. I don't sense any revenge in the plot.
How does the title do justice to the plot?
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The Sith used to rule the galaxy before the Republic and the Jedi Order overthrew them. This is what the Sith are taking revenge for in Star Wars: Revenge of the Sith.
As you could hear it from Master Windu and Palpatine's dialogue.
MACE WlNDU: The oppression of the Sith will never return. Your plot to regain control of the Republic is over . . .
PALPATINE: Once more, the Sith will rule the galaxy, and we shall have peace.
It is related to the Star Wars' Expanded Universe. Long story short:
They also used that title because the original title of Return of the Jedi was Revenge of the Jedi right up until just before the release. There was even merch made with the Revenge title that is pretty valuable if you still have some. At the last minute they decided that Jedi wouldn't be seeking revenge and changed it. I think they wanted to use the title Revenge of the Sith in homage to that.