Your "base AC" is your AC before any modifiers. However, it is not a defined game term at this time as it has little or no use outside of the Mage armor spell.

Such modifiers may include:

• Class based bonuses like from a Fighting style
• Magic bonuses from items
• Temporary bonuses from spells.
• Other things not included in this list.

So your "Base AC" is either your armor plus your ability (if applicable) plus your shield, or your natural armor plus your Dex or other stats as applicable. Also Mage Armor would set your "base" AC to 13+dex.

Your Base AC includes any applicable stat bonuses. So "base" in the case of Mage Armor is 13 + Dex, there is no additional bonus to that. A shield modifies your Base AC, and increase it by 2.

The armor + shield also includes your Dex for medium and light armors. So your Base AC in the following armor types is as follows:

• None: 10 + Dex
• Leather: 11 + Dex
• Hide: 12 + Dex (max 2)
• Chain: 16
• Chain + Shield: 18
• Mage Armor: 13 + Dex

Various other things set your base AC. But it's based on both your AC and the appropriate stat.

Lastly, there is no intent for there to be a difference between Unarmored Defenses and Mage Armor.

Wax Eagle: Is there a reason why the wording is inconsistent between Mage Armor and Unarmored Defenses? Mage Armor uses "Base AC"

Jeremy Crawford: The difference isn't intentional.

You only target one specific Effect, not all instances of an Effect of a certain type

You seem to be misunderstanding what it means by 'Magical Effect.'

Your interpretation of it is "A type of magical thing that is happening."

The actual meaning is "A Single, Discrete Magical Thing that is NOT a Creature or Object." You can't use Dispel Magic as an AoE Spell Purge to burn out all instances of a particular spell.

A Spell is a discrete Magical Effect

PHB201

When it refers to a Magical Effect, it is talking about things like Wall of Fire, Storm of Vengeance, Illusions, and so on. An individual instance of magic that is not attached to a creature or object. If that targeting option didn't exist, you couldn't Dispel effects like that, because a spell like Earthquake is neither a creature nor an object.

The target required by Dispel Magic is a specific target. Either 1 Creature, 1 Object, or 1 Magical Effect. You cannot target a type of magical effect.

And to give an actual Sage Advice quote...

Q: If dispel magic targets the magical effect from bless cast by a cleric, does it remove the effect on all the targets?

A: Dispel magic ends a spell on one target. It doesn’t end the same spell on other targets.