Dark side adept

"Even the most powerful of Dark Side Adepts believed that shrines of the sort existed only on Sith worlds remote from Coruscant, and even the most powerful of the Jedi believed that the power inherent in the shrine had been neutralized and successfully capped."

- Darth Sidious

A Dark Side Adept was someone with the power to use the dark side of the Force. They were often steeped in the lore of the dark side and opposed to those who used the light side, such as Jedi. While all Sith were Dark Side Adepts, the term was not exclusive to them, as non-Sith individuals such as the Grand Inquisitor were considered Dark Side Adepts as well.

Behind the scenes
In the continuity of Star Wars Legends, a Dark Side Adept was anyone using the dark side of the Force outside of the traditions of the Jedi or the Sith. Dark Side Adepts were referenced in passing in James Luceno's canon novel Tarkin.