Which still isn't a great definition, IMO.
They're fiends who aren't devils or demons -- neither of whom have a super-distinct identity (maybe they do in the 2025 MM, we'll see) beyond how much they like organization -- but sure look more or less like them and do awful things like them and live in horrific netherworlds like them ...
When 6E rolls round, it'd be great if we could get someone to be able to come up with a one-sentence description of each major monster type that isn't just recursively linking back to existing D&D lore or cosmology.
If we're going to have all of these different fiends, they should be be more than just big bags of hit points with pointy bits who are eeeevil.
As it is, the Blood War -- if that's necessary to carry forward -- would work just as well with a single category of monsters called Fiends, who war amongst themselves and have formed into two major camps.