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
But if we expand the acronym AC, "Base Armor Class" also appears: but again, just once, as far as I can see. This is in the equipment chapter, and armor and shields, and says:
The armor (and shield) you wear determines your base Armor Class
By this, it seems logical to assume that "base Armor Class" is the armor value, possibly plus 2 for a shield. But, in the case of Mage Armor, dexterity is specifically included in the "base AC". Is it intended that dexterity is added on top of that, for a total AC of 13 + dex modifier + dex modifier again?
If you look at the details for light, medium, and heavy armor, each of these specifies individually how Dex is treated (always added to Light, added up to +2 for Medium, and never factored in even if negative for Heavy). That sounds like the influence of Dex, if any, is included in the part of the armor, and therefore in your base Armor Class. But, on the other hand, these descriptions refer to "the base number from your armor type" — so is there a "base number" that's different from your "base AC"?
The question How does barkskin work? exemplifies how this confusion (for those of us used to more complicated AC from previous edition) trickles down: the question asks if something affects "total AC, Base AC or just armor AC". So my question, fundamentally, is: is "base AC" actually a special term in 5E, with a coherent meaning, or is it just some words that get used sometimes in describing armor class sometimes? If it is a definite term, what exactly does it include?