Some spells, such as Forbiddance, say that under certain circumstances, they last "Until Dispelled". Other spells, such as Mordenkainen's Private Sanctum, say that under certain circumstances, they become "Permanent".

What is the difference between these two terms? Does "Permanent" mean that the effect can never be dispelled?

The verbiage "Until Dispelled" and "Permanent" are functionally the same with regards to spell duration.

As Jeremy Crawford indicates here.

If the effect of a spell becomes permanent, it can be dispelled,
unless its description says otherwise.

Later this makes it into the official Sage Advice Compendium

Can permanent magical effects be dispelled? Or are they no longer considered magical effects once permanent?

If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise (such as in the wall of stone spell).

So unless a spell description says otherwise it is still an effect of the spell and dispel magic ends spells.

The logic behind RAW as I see it is that dispel magic ends spells. A spell effect that becomes permanent is still an effect created by a spell, thus can be ended with dispel magic unless more specific verbiage (e.g. the text of a specific spell) states otherwise.

No difference.

Despite being an old question, I'll have to put here the other perspective. According to JC,

• So the durations "Until Dispelled" and "Permanent" are functionally equivalent unless we houserule one of them?


• That's correct.

And according to Sage Advice,

• Can permanent magical effects be dispelled? Or are they
  no longer considered magical effects once permanent?


• If
  the effect of a spell becomes permanent, it can be dispelled,
  unless its description says otherwise (such as in the wall of
  stone spell).

Some spells do say "until dispelled", like Forbidance or True Polymorph, while others use the "permanent" nomenclature, like Private Sanctum. The discrepancy is strange, but it may have been an oversight from the designers. I think the idea is that, any magical effect created by a spell can be dispelled, thus falling in line with Dispel Magic's text.

Choose any creature, object, or magical effect within range. Any spell [...] on the target ends.

There are no assumptions regarding the duration of the effect, only that its spell (and thus effect) is terminated. Also consider spells like Wall of Stone, that claim

If you maintain your concentration on this spell for its whole duration, the wall becomes permanent and can’t be dispelled.

So, despite being permanent, they specify how they can't be dispelled. This seems another indication that permanent just means "infinite duration".

As a DM, maybe you can house-rule it so that it is much harder (or even impossible) to dispel permanent effects (similar to Artifact magical items), but at least according to the designers, it seems that, as you asked, there is no difference between "until dispelled" and "permanent".

RAW, yes it can be dispelled.

A Teleportation Circle would be a magical effect (it is the effect of a spell, after all). There is nothing in the spell description that says making it permanent changes this.

And Dispel Magic specifically says it can target a magical effect as you point out.

So this would work.

On the other hand, it seems a bit harsh for a year of work to be cancelled out by a single casting of Dispel Magic! As a DM I might declare that a Teleportation Circle made permanent is now a magic item rather than a magic effect and therefore not affected by Dispel Magic (or possibly only temporarily suppressible). But this would be a homebrew ruling.

A teleportation circle is the effect of a spell.

Effects of spells are magical effects.

So you can dispel a teleportation circle; the DC is 15. Arguably the wizard could cast the last teleportation circle at a higher level to boost the DC slightly.

However, it is worse than that.

Many major temples, guilds, and other important places have permanent teleportation circles inscribed somewhere within their confines. Each such circle includes a unique sigil sequence--a string of magical runes arranged in a particular pattern.

A permanent teleportation circle must have a unique sigil sequence -- a string of magical runes arranged in a particular pattern -- inscribed somewhere within their confines.

Thus, if you remove the unique sigil sequence, there is no teleportation circle there anymore. Whatever material the circle is inscribed on can be vandalized and the circle destroyed.

(Permanent Teleportation Circle) implies (Sigils Inscribed), thus not (Sigils Inscribed) implies not (Permanent Teleportation Circle). So either destroying the Sigils destroys the Circle, or the Sigils cannot be destroyed (which seems to be stretching things).

So Dispel Magic is just slightly faster than a mundane with a pickaxe.

The question the destroyer has to ask is, do they want to destroy a wizard-year of work and almost 20k of components invested? Or do they want to learn another destination.

I'm not sure how accurate my interpretation is, but I'd argue a permanent teleportation circle is not a magical effect any more than someone's death due to Power Word Kill is, and thus unless Dispel Magic could resurrect someone killed with Power Word Kill, I'd argue a permanent teleportation circle can't be dispelled either.

I have to disagree that the duration is permanent. The spell's duration is 1 round/6 seconds. The wording of the spell says you can "create a permanent teleportation circle", rather than extend the duration of anything. I'd argue that a "Permanent Teleportation Circle" is a completely separate thing from a Teleportation Circle, and that there's no magic holding one in place. I believe it to be more simply like a marker, perhaps a "dent" in the Weave so to speak. It doesn't teleport anything; rather, a normal Teleportation Circle does. A permanent circle simply serves as a marker for the spell to "lock on" to. The magic lasts 6 seconds each time the spell is used, but beyond that what is left is not a magical effect in my interpretation.