Long War
"We were made to conquer the galaxy, not to rot here in Hell and die upon our brothers' blades."
It's called the Long War because it truly is loooooooooooooonnnnnnnnnngggggg. That is not taking the mickey either. It has been going on for 10,000 years and sees no signs of abating. Well, sort of, anyway. Perhaps it is now more accurate to say that it HAD seen no signs of abating. From the perspective of certain a collective of edgy neerdowells, the destruction of Cadia was a lot of fun, and adding that big jelly splotch across the galactic map, thus ruining the day of all of the Imperium's cartographers (who'd worked very hard on that map) was a rip-roaring laugh. Unfortunately for both a Certain (H)Armless Failure, as well as his benefactors, the resulting commotion had the unintended side effect of waking up a very stern Super-Space-Roman, who takes great exception to both rabble-rousing and wanton disregard for maps, and has been handing out spankings to Chaos ever since. Worse was yet to come for Team Chaos though, because that big mess they made on the map which they were so proud of, also ended up revitalizing that Super-Space-Roman's Daddy, and delivering the galaxy at large a class-act presentation of Not As Planned so grand that Cegorach himself would smile approvingly (unless of course HE was somehow its screenwriter all along).
It is the ongoing conflict within Warhammer 40k between the forces of the Chaos Traitor Legions who turned to Chaos during the Horus Heresy and the mess of a super empire that is the Imperium. According to the traitors the war is still ongoing (hence long), but as far as the Imperium is concerned they won when they killed the traitor leader, routed his forces and pushed them back to the Eye of Terror, though even before the opening of the Great Rift they had suffered significant losses to the Traitor Legions' forces. However, that was offset by the subsequent return of Roboute Guilliman to help stabilize the Imperium, and for all the big mess caused by the Rift, it also resulted in Big-E receiving a massive fuckoff power boost strong enough to literally burn down a large part of Nurgle's Garden, which doesn't exactly bode well for the Ruinous Powers. How exactly does Disappointmaddon keep his job, again? Add to that the incoming threat posed by the new Tyranid fleet, the even more recent return of the motherfucking Lion, but most of all: the Necrons starting to wake up in greater numbers (who, if unified, effectively can shut the party down for everyone simultaneously)... Yeah, Chaos is going to be getting some awfully loud and disruptive outside interference interrupting their game which they're going to have a hard time countering.
It is a rather confusing term since no one is quite clear who is and isn't part of the conflict, what battles are covered under it and various other details about it. Do the Space Marines that have turned to Chaos since the heresy count as being part of the Long War? Do the Lost and the Damned and the Daemons of Chaos count as taking part as well? Does every conflict fought between Chaos and the Imperium automatically count as part of the Long War? It tends to be one of those terms that people toss around casually without referencing it further. This could be GW's stubborn refusal to let too much fluff out of its forbidden depths. Then again, it's very much possible that, in universe, they aren't really sure what they mean when they use the term either. It's...well, the Chaos Marines buzzword for how much they hate the Imperium.
Actually there is likely a very good reason the Long War has gone on as long as it has: the leadership of a certain armless failure....
But more seriously, the Chaos Gods are more interested in war itself as a process rather than its final result. Especially given that according to the Cabal the ultimate Chaos victory would be their undoing. I mean eternal war spreads eternal bloodshed and carnage, massive death tolls and epidemics, struggles for power and epic level mind games between the leadership of opposing factions and leads warriors involved into new peaks of sensual experience. So they subtly undermine their servants' plans if they come too close to ultimate victory, especially since they know that Abaddon's chief loyalty is to himself alone and would turn on them immediately after finishing off the Imperium. Just as Planned.