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 Inquisitorius and Snoke, 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 2014 canon novel Tarkin. Lucasfilm Story Group member Matt Martin later clarified on Twitter that the book, which had spelled the term as "Dark Side Adept," had made a mistake, and that it shouldn't be capitalized.