Definition: The title "Darth," indicating a Sith, first appeared with Darth Vader in Episode IV: A New Hope. At first it was treated almost like a first name; Obi-Wan Kenobi calls Vader simply "Darth" in the film, and Splinter of the Mind's Eye refers to him as "Lord Darth Vader." As the Star Wars universe grew, "Darth" became a title used by Dark Lords of the Sith.
In-universe: The etymological origins of the title Darth are unclear and disputed. It may be as simple as a contraction of "Dark Lord of the Sith." Alternatively, it may be derived from Daritha, the word for "Emperor" in the language of the Rakata, a humanoid species who controlled a powerful empire thousands of years before the Galactic Republic. It may also come from the Rakatan darr tah, meaning "triumph over death" (i.e., immortality, the ultimate goal of the Sith) or "conquest through death" (i.e., the destruction of one's enemies).
The title Darth was in use by the Sith at least as early as the Sith Lord Darth Drear, around 4,645 BBY. At the time of the New Jedi Order, the earliest known Sith to use the title were Darth Revan and Darth Malak, whose rediscovery of the Rakata lent credence to the Rakatan theory of origin. Because the term was used before Revan and Malak, however, it may come from a different source altogether.
In general, Sith adopted a new name when they took the title Darth, signifying the transformation they underwent on their path to the dark side. These new Sith names reveal the Sith's personality or role, usually with a complete lack of subtlety; for example, Darth Sidious, Darth Bane, and Darth Plagueis. A few Sith -- most notably, Darth Revan -- kept their given names.
The title Darth was not used in all eras of the Sith Order. By the time of Darth Bane, around 1,000 BBY, Darth had fallen out of use in order to prevent conflict among the many Sith, and most Sith used the title Dark Lord. Bane revived the title when he created the Rule of Two, using it for both himself and his apprentice, Darth Zannah. This tradition continued for over a thousand years. In Darth Krayt's One Sith, founded around 30 ABY, the title Darth was only granted to Sith who proved both their skill in the Force and their loyalty to Darth Krayt.
- Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham (2007)