I am not 100% but yes a Sith is a dark Jedi. However, the Sith is a dark side religion, where they have one master and one apprentice.

A Dark Jedi is a Jedi who follows th Dark Side of the force, but may not necessarily be part of the Sith.

Dark Jedi were the origins of the Sith. It’s basically any Jedi who has rejected the Light Side for the Dark.

Sith are something entirely different. Sith are essentially the anti-Jedi, because the Jedi were anti-Dark Jedi first.

I always figured Dark Jedi were like Grey Jedi, but a little less goodie-goodie and a bit more stealy-stealy. LOL! Like, not a Sith, but just their own independent thing. Only in it for themselves. But that was just my guess, I could be totally wrong.

Pretty much what Pete Hobson said.

Sith are their own group, independent from the Jedi, and follow the Dark Side.

Dark Jedi are Jedi who have left the order to follow the Dark Side. Some Dark Jedi work for or become Sith, but they don't have to

A Sith is a member of a Sith organisation (like the Order of the Sith Lords), or alternatively (in Legends) a member of the Sith species. A Dark Jedi is any Force user who has devoted themselves to the dark side, regardless of their affiliation.

I think dark jedis are jedis who have turned to the dark side, while sith are just dark siders from the start. Not sure tho