The phrase "base AC" appears in the D&D 5E basic rules exactly once, in the description of the Mage Armor spell. This says: The target’s base AC becomes 13 + its Dexterity modifier. But ...
I was poking around answering for this question and noticed something odd. It doesn't seem to be possible except in the case of the Barbarian to naturally get your AC over 20 without the use of magic ...
Mage Armor sets the target's base AC to 13+Dex. Shield gives you a +5 bonus to AC. If you cast both on yourself, does that give you an effective AC of 18+Dex while both spells are in effect?
Do you get a proficiency bonus to AC when wearing armor you are proficient with?