How does the title "Revenge of the Sith" suit the plot?

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:

• There was only Jedi Order
• A group of radical Jedi wants to use negative emotion such as anger, envy, and selfishness as a source of the force
• This group of radical Jedi was kicked out and exiled onto a lonely planet outside of the Old Republic
• They started calling themselves Sith and formed the Sith Order, a sworn enemy of the Jedi Order
• The Sith Order and the Jedi Order have fought many battles, for several times, the Sith Order almost won but due to in-fight, the Jedi managed to defeat the sith in the end.
• A Sith Lord invented an idea, the Rule of Two, there could be only two Sith Lords at any time, a teacher and a student. This way, the Sith could hide themselves better from the Jedi order.
• When the Sith finally took control of the republic, the clone army, and executed the Order 66; they finally defeated the Jedi, it was their revenge.

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.