"A Sith who advances in the order’s hierarchy will eventually be elevated to Sith Lord. His or her name is preceded by "Lord" in formal address. In the modern Empire, "Dark Lord" is synonymous with this term. An acolyte who completes the trials may be accepted as the apprentice of a Sith Lord."
Sith Lord is a rank that is conferred on a powerful Sith who has considerable knowledge of the dark side. The term "Sith Lord" generally encompassed members of both genders, although some female Sith are styled Sith Lady, Lord referred to a high-ranking member of the Sith race's society. As the Sith civilization progressed, many claimed the title and earn the superior rank of Sith'ari: the Sith Overlord: Adas and Dathka Graush were such Overlords. it is the second highest rank in the Sith Order.