I'm trying to figure out an appropriate word to describe a magical effect of unknown origin in a D&D game. I can't call it a spell or spell effect since it's not known whether the effect is caused by a spell or some other kind of magic. The most obvious term that comes to mind is "enchantment", but in the context of D&D, this term is reserved specifically for magic that affects the minds of others, so it is not suitable as a general term for any magical effect. So what would an appropriate term be for a persistent magical effect?
Unless you're explicitly searching for a synonym/alternative, I think 'magical effect' itself is appropriate- it covers both spell and non-spell cases. In addition, several game elements (in 5e, at least) refer to them as such:
Choose one creature, object, or magical effect within range. [...]
Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere.
Spells and other magical effects, such as magic missile and charm person, that target a creature or an object in the sphere have no effect on that target.
You change the way the target appears to spells and magical effects [...]
Enchantment has two meanings in D&D.
The narrow one is a mind affecting spell.
The more common meaning is magic imbued in an object or creature. A magic sword has an enchantment on it. You could even say that the narrow meaning is just a specific case of the broader meaning because the mind affecting spell is a spell that imbues a creature with an effect for a specific amount of time.
For the other part of your question, "permanent" is the term for a spell that does not have an end point to its duration but is ongoing. This is different from an Instant spell that makes a change to an object where the change is then permanent (the damage from a fireball persists after the spell and cannot be dispelled).
I would refer to it based on how the effect manifests itself. For example you could call it a dome, cloud, haze, mist, field, emanation, or any other more specific or appropriate adjective.