In D&D 5e, the spell Dispel Magic says
Choose one creature, object, or magical effect within
The rest of the spell description explains that spells "on the target" of a level lower than that at which Dispel Magic was cast automatically end, and those of a higher level require a check.
This makes perfect sense when targeting creatures or objects which may have spells on them. But what does it mean to target a magical effect?
Does it mean that you can choose to target just one specific ongoing spell effect instead of stripping the whole shebang (clearing a negative spell from an ally with positive spells also in place, or a positive spell from an enemy who is under attack from a negative spell)?
Or, is this only for the special case of certain spells (like Imprisonment) which have effects which specifically mention being targeted by Dispel Magic, and since no effect is described otherwise? (This is a literal reading: a spell is a magical effect, but does not have spells on it, so the Dispel Magic effect has nothing to act on.)
Or — and I think this is a stretch — can magical effects which are not spells be dispelled by Dispel Magic? If so, what are the mechanics?
Dispel Magic Only Ends Spells
A magical effect that is not the result of an ongoing spell cannot be dispelled.
Jeremy Crawford has issued several Twitter rulings to this effect.
In this Tweet:
and this one:
and this series of Tweets:
Also note that Dispel Magic can only end spells with an ongoing duration. It cannot remove the lingering effects of an instantaneous spell. PHB P. 203:
The March 2016 Sage Advice Column covers Dispel Magic as well. It reiterates that Dispel magic cannot end magical effects that are not the result of a spell, such as a vampire's Charm effect. It also reiterates the rule about Dispel Magic not working on the results of a spell with an instantaneous effect. It adds that a readied Dispel Magic cannot be used in place of a Counterspell, though in some limited circumstances, it may be almost as good.
To answer your actual question, in this context, targeting a magical effect clearly means you can target just one specific ongoing spell effect instead of stripping everything on a particular creature.