Order of the Sith Lords
Sith Lord is a title of distinction carried by former Grand Masters as well as select members deemed worthy of the elevation, often but not limited to a Deputy Grand Master completing their apprenticeship as the Shadow Hand. Members of the Sith Lord title are permitted to style themselves as Lord, Lady, or Darth in homage to the ancient Sith rulers.
History
Originally, Sith Lord referred to a high-ranking member of the Sith race's society. As the Sith civilization progressed, many claimed the title; Adas was one such lord. While obviously referring to a master of Sith knowledge, the title of "Sith Lord" (or sometimes "Sith Lady" if referring to a female Dark Lord) seemed to be used somewhat ambiguously to refer to various things, including all Sith, the Dark Lords of the Sith, Sith Masters in Darth Bane's reformed order, or as a rank akin to that of the Jedi Master for the Jedi Order. Several Sith Lords also held the title Darth.
Dark Jedi Brotherhoood
Italics refers to previous Dark Lord of the Sith bearers