Dark Apostle
The Traitor Legions have a lot of things that are basically "Evil Knockoffs!" to stuff the Loyalists get. And even the Chaplains aren't exempt. Whereas the Chaplains focus on treating the spiritual and mental health of their fellows and ensuring they stay loyal to the God-Emperor of Mankind, their Traitor Counterparts, the Dark Apostles, are twisted priests and seers of the Chaos Gods, leading the profane rituals that serve and honor their blasphemous masters.
The Dark Apostles proper are traitor Chaplains of the Word Bearers Legion. Indeed, they are the true leaders of the Word Bearers even in their current state; the warbands, or Hosts, of that Traitor Legion all operate under the authority of the Dark Council, a collection of the oldest Dark Apostles who lead the Word Bearers while Lorgar spends his time thinking about the Chaos Gods. This also makes the Word Bearers legion possibly the most dangerous of all, seeing as they are still unified (basically).
However, the name is also used for other Chaos Marine leaders, serving as a generic term for charismatic preachers and zealous orators among the servants of Chaos - in essence, a Champion gifted with leadership skills instead of pure, raw killpower, especially one who focuses on spreading the worship of Chaos (usually, but not always, Chaos Undivided) as a corrupt priest-figure.
In 3rd edition, when Chapter Approved released the Word Bearers as part of the Codex Astartes series, it stated that Dark Apostles are the only Traitor Legion Chaplains who fell from the Emperor's light, forcing all other Traitor Legions to massacre their Chaplains. With the edition change and especially the Horus Heresy novels, that fluff has been retconned; in the days preceding the Heresy, the Emperor championed the Imperial Truth, which basically goes "there are no gods and you shall not worship anything, not even me - are you listening, Lorgar?!" As a result, no other Legion of the time HAD Chaplains, except for the Word Bearers; they were just being phased into place (as, essentially, Space Marine Commissars rather than warrior-priests) when the Heresy broke out, and became adopted by the Loyalists afterward. This change is why Dark Apostles went from a Word Bearers-exclusive unit to something any Chaos Marine army can field.