What is the difference, if any, between “Permanent”, and “Until Dispelled”?

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?

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.

• 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".

• 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. It is absolutely amazing that WotC can put out both mtg, a game with an extremely tight set of rules, and DnD, which is so very much the opposite. They use ambiguous / synonymous terms, when it gets pointed out that it is confusing they only errata it some of the time. Absolute Insanity! – Shane Nov 28 '18 at 6:41

Unfortunately to the best of my knowledge this isn't specified in any detail in the books. However, the language itself isn't meant to hide anything so:

"Until Dispelled" means that the caster retains some hold on the magic after its been cast, and can still dispel it at any time. At the same time, however, the spell does not require active concentration once cast, so the caster can do anything else he wishes while the spell is in effect.

"Permanent" however, means that once cast, the caster cannot simply turn it off. The Permanent duration bestows no special resistance to dispelling however, but even the caster would need to cast a spell halt the effect.

As always with 5th edition, YMMV.

• I wasn't aware that casters could dispel their own spells at will, unless the spell required concentration. Could you give a citation for that? – Strill Sep 11 '15 at 2:23
• Fire Shield, among other spells with durations, lists that it can be dismissed before its 10-minute duration is up. However, as I stated in my first paragraph the is no specific definition of "Until Dispelled" in the books. Most games I've played in, the assumption was the caster could always end his own spells early. But again YMMV. – Brian_Drozd Sep 11 '15 at 2:41
• Guards and Wards, Mordekainen's Private Sanctum, Nystul's Magic Aura, and Teleportation Circle all have increased duration if you cast them in the same location repeatedly. – Strill Sep 11 '15 at 2:43