Darth (Galaxy of the Force)

Darth was a title given to Sith in the Sith Order and the Empire which preceded their Sith name or in very rare cases, their birth name.

Canon
Here is the canon info of the Darth title.

Usage in the Sith
A Sith would recieve their Darth title when they would achive the rank of Sith Warrior or Sith Acolyte. The Darth name was normally chosen by the one reciving the name early in the history of the Sith Order but later on after the death of the leader of the Order, the masters of the apprentice decided the name for them. In some cases, the apprentice would use their birth name as their Darth name but by the time Darth Matas took over the Empire, birth names were not allowed as new Darth names anymore.